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Precipitation

Precipitation

Precipitation is water in either solid or liquid form that falls from Earth's atmosphere. Major forms of precipitation include rain, snow, and hail. When air is lifted in the atmosphere, it expands and cools. Cool air cannot hold as much water in vapor form as warm air, and the condensation of vapor into droplets or ice crystals may eventually occur. If these droplets or crystals continue to grow to large sizes, they will eventually be heavy enough to fall to the earth's surface.

Precipitation in liquid form includes drizzle and raindrops. Raindrops are on the order of a millimeter (one thousandth of a meter) in radius, while drizzle drops are approximately a tenth of this size. Important solid forms of precipitation include snowflakes and hailstones. Snowflakes are formed by aggregation of solid ice crystals within a cloud, while hailstones involve supercooled water droplets and ice pellets. They are denser and more spherical than snowflakes. Other forms of solid precipitation include graupel and sleet (ice pellets). Solid precipitation may reach Earth's surface as rain if it melts as it falls. Virga is precipitation that evaporates before reaching the ground.

Precipitation forms differently depending on whether it is generated by warm or cold clouds . Warm clouds are defined as those that do not extend to levels where temperatures are below 32°F (0°C), while cold clouds exist at least in part at temperatures below 32°F (0°C). Temperature decreases with height in the lower atmosphere at a moist adiabatic rate of about 3.3°F per 3,281 ft (6°C per 1,000 m), on average. High clouds, such as cirrus, are therefore colder and more likely to contain ice. As discussed below, however, temperature is not the only important factor in the formation of precipitation.

Even the cleanest air contains aerosol particles (solid or liquid particles suspended in the air). Some of these particles are called cloud condensation nuclei, or CCN, because they provide favorable sites on which water vapor can condense. Air is defined to be fully saturated, or have a relative humidity of 100%, when there is no net transfer of vapor molecules between the air and a plane (flat) surface of water at the same temperature. As air cools, its relative humidity will rise to 100% or more, and molecules of water vapor will bond together, or condense, on particles suspended in the atmosphere. Condensation will preferentially occur on particles that contain water soluble (hygroscopic) material. Types of particles that commonly act as CCN include sea-salt and particles containing sulfate or nitrate ions; they are typically about 0.0000039 in (0.0001 mm) in radius. If relative humidity remains sufficiently high, CCN will grow into cloud droplets 0.00039 in (0.01 mm) or more in size. Further growth to precipitation size in warm clouds occurs as larger cloud droplets collide and coalesce (merge) with smaller ones.

Although large quantities of liquid water will freeze as the temperature drops below 32°F (0°C), cloud droplets sometimes are supercooled; that is, they may exist in liquid form at lower temperatures down to about 40°F (40°C). At temperatures below 40°F (40°C), even very small droplets freeze readily, but at intermediate temperatures (between 40 and 32°F or 40 and 0°C), particles called ice nuclei initiate the freezing of droplets. An ice nucleus may already be present within a droplet, may contact the outside of a droplet and cause it to freeze, or may aid in ice formation directly from the vapor phase. Ice nuclei are considerably more rare than cloud condensation nuclei and are not as well understood.

Once initiated, ice crystals will generally grow rapidly because air that is saturated with respect to water is supersaturated with respect to ice; i.e., water vapor will condense on an ice surface more readily than on a liquid surface. The habit, or shape, of an ice crystal is hexagonal and may be plate-like, column-like, or dendritic (similar to the snowflakes cut from paper by children). Habit depends primarily on the temperature of an ice crystal's formation. If an ice crystal grows large enough to fall through air of varying temperatures, its shape can become quite intricate. Ice crystals can also grow to large sizes by aggregation (clumping) with other types of ice crystals that are falling at different speeds. Snowflakes are formed in this way.

Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes another object. If a supercooled droplet collides with an ice crystal, it will attach itself to the crystal and freeze. Supercooled water that freezes immediately will sometimes trap air, forming opaque (rime) ice. Supercooled water that freezes slowly will form a more transparent substance called clear ice. As droplets continue to collide with ice, eventually the shape of the original crystal will be obscured beneath a dense coating of ice; this is how a hailstone is formed. Hailstones may even contain some liquid water in addition to ice. Thunderstorms are dramatic examples of vigorous mixed clouds that can produce high precipitation rates. The electrical charging of precipitation particles in thunderstorms can eventually cause lightning discharges.

Precipitation reaching the ground is measured in terms of precipitation rate or precipitation intensity. Precipitation intensity is the depth of precipitation reaching the ground per hour, while precipitation rate may be expressed for different time periods. Typical precipitation rates for the northeastern United States are 23 in (5080 mm) per month, but in Hilo, Hawaii, 49.9 in (127 cm) of rain fell in March 1980. Average annual precipitation exceeds 80 in (200 cm) in many locations. Because snow is less compact than rain, the mass of snow in a certain depth may be equivalent to the mass of rain in only about one-tenth that depth (i.e., one inch of rain contains as much water as about 10 in [25 cm] of snow). Certain characteristics of precipitation are also measured by radar and satellites.

The earth is unique in our solar system in that it contains water, which is necessary to sustain life as we know it. Water that falls to the ground as precipitation is critically important to the hydrologic cycle , the sequence of events that moves water from the atmosphere to the earth's surface and back again. Some precipitation falls directly into the oceans , but precipitation that falls on land can be transported to the oceans through rivers or underground in aquifers. Water stored in this permeable rock can take thousands of years to reach the sea. Water is also contained in reservoirs such as lakes and the polar ice caps, but about 97% of the earth's water is contained in the oceans. The sun's energy heats and evaporates water from the ocean surface. On average, evaporation exceeds precipitation over the oceans, while precipitation exceeds evaporation over land masses. Horizontal air motions can transfer evaporated water to areas where clouds and precipitation subsequently form, completing the circle which can then begin again.

The distribution of precipitation is not uniform across the earth's surface, and varies with time of day, season and year. The lifting and cooling that produces precipitation can be caused by solar heating of the earth's surface, or by forced lifting of air over obstacles or when two different air masses converge. For these reasons, precipitation is generally heavy in the tropics and on the upwind side of tall mountain ranges. Precipitation over the oceans is heaviest at about 7°N latitude (the intertropical convergence zone), where the tradewinds converge and large thunderstorms frequently occur. While summer is the "wet season" for most of Asia and northern Europe , winter is the wettest time of year for Mediterranean regions and western North America . Precipitation is frequently associated with large-scale low-pressure systems (cyclones) at mid-latitudes.

Precipitation is obviously important to humankind as a source of drinking water and for agriculture. It cleanses the air and maintains the levels of lakes, rivers, and oceans, which are sources of food and recreation. Interestingly, human activity may influence precipitation in a number of ways, some of which are intentional, and some of which are quite unintentional. These are discussed below.

The irregular and frequently unpredictable nature of precipitation has led to a number of direct attempts to either stimulate or hinder the precipitation process for the benefit of humans. In warm clouds, large hygroscopic particles have been deliberately introduced into clouds in order to increase droplet size and the likelihood of collision and coalescence to form raindrops. In cold clouds, ice nuclei have been introduced in small quantities in order to stimulate precipitation by encouraging the growth of large ice crystals; conversely, large concentrations of ice nuclei have been used to try to reduce numbers of supercooled droplets and thereby inhibit precipitation formation. Silver iodide, which has a crystalline structure similar to that of ice, is frequently used as an ice nucleus in these "cloud seeding" experiments. Although certain of these experiments have shown promising results, the exact conditions and extent over which cloud seeding works and whether apparent successes are statistically significant is still a matter of debate.

Acid rain is a phenomenon that occurs when acidic pollutants are incorporated into precipitation. It has been observed extensively in the eastern United States and northern Europe. Sulfur dioxide, a gas emitted by power plants and other industries, can be converted to acidic sulfate compounds within cloud droplets. In the atmosphere, it can also be directly converted to acidic particles, which can subsequently act as CCN or be collected by falling raindrops. About 70 megatons of sulfur is emitted as a result of human activity each year across the planet. (This is comparable to the amount emitted naturally.) Also, nitrogen oxides are emitted by motor vehicles, converted to nitric acid vapor, and incorporated into clouds in the atmosphere.

Acidity is measured in terms of pH , the negative logarithm of the hydrogen ion concentration; the lower the pH, the greater the acidity. Water exposed to atmospheric carbon dioxide is naturally slightly acidic, with a pH of about 5.6. The pH of rainwater in remote areas may be as low as about 5.0 due to the presence of natural sulfate compounds in the atmosphere. Additional sulfur and nitrogen containing acids introduced by anthropogenic (human-induced) activity can increase rainwater acidity to levels that are damaging to aquatic life. Recent reductions in emissions of sulfur dioxide in the United Kingdom have resulted in partial recovery of some affected lakes.

Recent increases in anthropogenic emissions of trace gases (for example, carbon dioxide, methane, and chloroflourocarbons) have resulted in concern over the so-called greenhouse effect . These trace gases allow energy in the form of sunlight to reach the earth's surface, but "trap" or absorb the infrared energy (heat) that is emitted by the earth. The heat absorbed by the atmosphere is partially re-radiated back to the earth's surface, resulting in warming. Trends in the concentrations of these greenhouse gases have been used in climate models (computer simulations) to predict that the global average surface temperature of the earth will warm by 3.610.8°F (26°C) within the next century. For comparison, the difference in average surface temperature between the Ice Age 18,000 years ago and present day is about 9°F (5°C).

Greenhouse warming due to anthropogenic activity is predicted to have other associated consequences, including rising sea levels and changes in cloud cover and precipitation patterns around the world. For example, a reduction in summertime precipitation in the Great Plains states is predicted by many models and could adversely affect crop production. Other regions may actually receive higher amounts of precipitation than they do currently. The level of uncertainty in these model simulations is fairly high, however, due to approximations that are made. This is especially true of calculations related to aerosol particles and clouds. Also, the natural variability of the atmosphere makes verification of any current or future trends extremely difficult unless actual changes are quite large.

As discussed above, gas-phase pollutants such as sulfur dioxide can be converted into water-soluble particles in the atmosphere. Many of these particles can then act as nuclei of cloud droplet formation. Increasing the number of CCN in the atmosphere is expected to change the characteristics of clouds. For example, ships' emissions have been observed to cause an increase in the number of droplets in the marine stratus clouds above them. If a constant amount of liquid water is present in the cloud, the average droplet size will be smaller. Higher concentrations of smaller droplets reflect more sunlight, so if pollution-derived particles alter clouds over a large enough region, climate can be affected. Precipitation rates may also decrease, since droplets in these clouds are not likely to grow large enough to precipitate.

See also Air masses and fronts; Atmospheric chemistry; Atmospheric circulation; Atmospheric composition and structure; Atmospheric pollution; Atmospheric pressure; Blizzards and lake effect snows; Clouds and cloud types; Greenhouse gases and greenhouse effect; Rainbow; Seasonal winds; Tropical cyclone; Water pollution and biological purification; Weather forecasting methods; Weather forecasting

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precipitate

pre·cip·i·tate • v. / priˈsipəˌtāt/ [tr.] 1. cause (an event or situation, typically one that is bad or undesirable) to happen suddenly, unexpectedly, or prematurely: the incident precipitated a political crisis. ∎  [tr.] cause to move suddenly and with force: suddenly the ladder broke, precipitating them down into a heap. ∎  (precipitate someone/something into) send someone or something suddenly into a particular state or condition: they were precipitated into a conflict for which they were quite unprepared. 2. (usu. be precipitated) Chem. cause (a substance) to be deposited in solid form from a solution. ∎  cause (drops of moisture or particles of dust) to be deposited from the atmosphere or from a vapor or suspension. • adj. / priˈsipətət/ done, made, or acting suddenly or without careful consideration: I must apologize for my staff—their actions were precipitate. ∎  (of an event or situation) occurring suddenly or abruptly: a precipitate decline in cultural literacy. • n. / priˈsipətət; -əˌtāt/ Chem. a substance precipitated from a solution. DERIVATIVES: pre·cip·i·ta·ble / priˈsipətəbəl/ adj. pre·cip·i·tate·ly / priˈsipətətlē/ adv. pre·cip·i·tate·ness / priˈsipətətnəs/ n.

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precipitation (in chemistry)

precipitation, in chemistry, a process in which a solid is separated from a suspension, sol, or solution. In a suspension such as sand in water the solid spontaneously precipitates (settles out) on standing. In a sol the particles are precipitated by coagulation. A solute (dissolved substance) may be precipitated from a solution by several means. A solution of salt may be concentrated by evaporation until the salt crystallizes. When a saturated solution of sugar is cooled, sugar crystals form. The addition of a solution of silver nitrate to a solution containing chloride ions results in the formation of insoluble silver chloride: AgNO3+Cl-→NO3-+AgCl↓. In each case the precipitate formed may settle out spontaneously or may be collected by filtration or centrifugation. It is often difficult to obtain a pure substance by a single precipitation, and a substance may be further purified by reprecipitation after it has been redissolved. The term precipitation is also applied to the separation of particles of a solid or liquid suspended in a gas.

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precipitation

precipitation In meteorology, all forms of water particles, whether liquid or solid, that fall from the atmosphere to the ground. Distinguished from cloud, fog, dew, and frost, precipitation includes rain, drizzle, snow and hail. Measured by rain and snow gauges, the amount of precipitation is expressed in millimetres or inches of liquid water depth. In chemistry, the formation of an insoluble solid in a liquid by a reaction in the liquid between two or more soluble substances. This is the opposite of dissolving. Precipitation is used to create insoluble salts; precipitation reactions are employed to recognize certain ions.

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precipitation

precipitation
1. In meteorology, all the forms in which water (H2O) falls to the ground as rain, sleet, snow, hail, drizzle, or other more specialized forms, and also the amounts measured. Sometimes precipitation seen falling from clouds evaporates before reaching the ground.

2. The process of depositing dust or other substances (pollution) from the air.

3. The deposition of solid particles out of a supersaturated solution.

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precipitation

pre·cip·i·ta·tion / priˌsipəˈtāshən/ • n. 1. Chem. the action or process of precipitating a substance from a solution. 2. rain, snow, sleet, or hail that falls to the ground. 3. archaic the fact or quality of acting suddenly and rashly: Cora was already regretting her precipitation.

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precipitation

precipitation
1. All the forms in which water falls to the ground (i.e. as rain, sleet, snow, hail, drizzle, or other more specialized forms) and also the amounts measured. Sometimes precipitation seen falling from clouds evaporates before reaching the ground.

2. The deposition of dust or other substances (e.g. pollutants) from the air.

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precipitate

precipitate Formation of an insoluble solid in a liquid either by direct reaction or by varying the liquid composition to diminish the solubility of a dissolved compound.

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precipitation (in meteorology)

precipitation, in meteorology, condensed moisture that falls to the surface of the earth in the form of rain, sleet, snow, hail, frost, or dew.

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precipitate

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Precipitation

Precipitation

Chemistry

Meteorology

Types of precipitation

Formation of precipitation

Precipitation formation in warm clouds

Precipitation formation in cold clouds

Measurement of precipitation

Hydrologic cycle

Human influences on precipitation

Cloud seeding

Acid rain

Greenhouse effect

Effects of particulate pollution on cloud microphysics

Resources

Precipitation is defined both in chemistry and in meteorology. Precipitation, in chemistry, is a process that causes dissolved substances to separate from a solution as a solid. The resulting solid, from a precipitation reaction, is referred to as a precipitate. Precipitation reactions are used for such industrial applications as the production of salts; in scientific applications such as decanting and centrifuging; and within natural processes such as that found within sedimentary rocks. Precipitation, in meteorology, describes a process that takes place in the atmosphere, the condensation of water vapor to form rain droplets, snow, or hail. Both definitions will be discussed in more detail below.

Chemistry

Precipitation is a process that causes dissolved substances to separate from a solution as a solid. The resulting solid, from a precipitation reaction, is

referred to as a precipitate. Precipitation reactions are used for such industrial applications as the production of salts; in scientific applications such as decanting and centrifuging; and within natural processes such as that found within sedimentary rocks. In terms of physical processes, precipitation is the opposite of dissolution. In chemical reactions, ionic compounds that dissociate (break apart) in solution are termed ionic salts. Any resulting dissociated components (e.g., ions) that do not contribute to the formation of the precipitate are termed spectators (e.g., spectator ions). The components that react to form the precipitate are termed the precipitates constituent or contributing components.

Whether a precipitate will form depends on both the solute concentration and its saturation solubility at the specified temperature; that is, the maximum amount of a substance that can be dissolved in the given solvent. When an ionic solute is dissolved in water, equilibrium is established between the solid phase and the various hydrated ionic species. At equilibrium, the rate of formation of the dissociated ions exactly equals the rate of their recombination to form the solid solute. This equilibrium is governed mathematically by the solubility product, Ksp, which in the case of silver chloride, is given by AgCl(solid) Ag+(aq) + Cl-(aq) Ksp= [Ag+][Cl-] the product of the molar concentrations of the silver and chloride ions. Precipitation occurs whenever the numerical value of [Ag+][Cl-] is larger than the solubility product. This happens at supersaturation and the net reaction favors recombination rather than dissociation of components in solution. For precipitation to occur there must be at least a slight supersaturation of the substance in solution.

Pollutants or impurities in solution also contribute to components in solution and, therefore, can affect the rates and type of precipitation that occur. Selective precipitation is a process that separates ions out of a solution by deliberate creation of new precipitates introduced to solution. Selective precipitation was the basis behind the qualitative analysis scheme that was used years ago to identify which ions were present in unknown aqueous solutions.

A familiar example of a precipitation reaction is that observed in the limewater test for carbon dioxide; the formation of the characteristic milky-white precipitate. Before the wastewater is disinfected and discharged, phosphorous is removed by chemical precipitation through reactions with aluminum, iron, calcium, and clay minerals in soils.

Covalent compounds also precipitate when the solution becomes supersaturated. In the case of organic compounds, this action may occur when two species react to form a less soluble species. A simple compound in solution can precipitate if the solvent evaporates sufficiently, if the solution is cooled, or if a nonsolvent is added to the solution to reduce the solutes solubility. For example, the organic compound phenanthrene can be precipitated from an ethanolic solution by the addition of water. Phenanthrene is considerably more soluble in ethanol than in water.

Meteorology

Precipitation also describes a different process that takes place in the atmosphere, the condensation of water vapor to form rain droplets, snow, or hail. In meteorology, precipitation is water in either solid or liquid form that falls in Earths atmosphere. Major forms of precipitation include rain, snow, and hail. When air is lifted in the atmosphere, it expands and cools. Cool air cannot hold as much water in vapor form as warm air, and the condensation of vapor into droplets or ice crystals may eventually occur. If these droplets or crystals continue to grow to large sizes, they will eventually be heavy enough to fall to Earths surface.

Types of precipitation

Precipitation in liquid form includes drizzle and raindrops. Raindrops are on the order of a millimeter (one thousandth of a meter) in radius, while drizzle drops are approximately a tenth of this size. Important solid forms of precipitation include snowflakes and hailstones. Snowflakes are formed by aggregation of solid ice crystals within a cloud, while hailstones involve supercooled water droplets and ice pellets. They are denser and more spherical than snowflakes. Other forms of solid precipitation include graupel and sleet (ice pellets). Solid precipitation may reach Earths surface as rain if it melts as it falls. Virga is precipitation that evaporates before reaching the ground.

Formation of precipitation

Precipitation forms differently depending on whether it is generated by warm or cold clouds. Warm clouds are defined as those that do not extend to levels where temperatures are below 32°F (0°C), while cold clouds exist at least in part at temperatures below 32°F (0°C). Temperature decreases with height in the lower atmosphere at a moist adiabatic rate of about 3.3°F per 3,281 ft (1.8°C per 1,000 m), on average. High clouds, such as cirrus, are therefore colder and more likely to contain ice. As discussed below, however, temperature is not the only important factor in the formation of precipitation.

Precipitation formation in warm clouds

Even the cleanest air contains aerosol particles (solid or liquid particles suspended in the air). Some of these particles are called cloud condensation nuclei, or CCN, because they provide favorable sites on which water vapor can condense. Air is defined to be fully saturated, or have a relative humidity of 100%, when there is no net transfer of vapor molecules between the air and a plane (flat) surface of water at the same temperature. As air cools, its relative humidity will rise to 100% or more, and molecules of water vapor will bond together, or condense, on particles suspended in the atmosphere. Condensation will preferentially occur on particles that contain water soluble (hygroscopic) material. Types of particles that commonly act as CCN include sea-salt and particles containing sul-fate or nitrate ions; they are typically about 0.0000039 in (0.0001 mm) in radius. If relative humidity remains sufficiently high, CCN will grow into cloud droplets 0.00039 in (0.01 mm) or more in size. Further growth to precipitation size in warm clouds occurs as larger cloud droplets collide and coalesce (merge) with smaller ones.

Precipitation formation in cold clouds

Although large quantities of liquid water will freeze as the temperature drops below 32°F (0°C), cloud droplets sometimes are supercooled; that is, they may exist in liquid form at lower temperatures down to about -40°F (-40°C). At temperatures below -40°F (-40°C), even very small droplets freeze readily, but at intermediate temperatures (between -40 and 32°F or -40 and 0°C), particles called ice nuclei initiate the freezing of droplets. An ice nucleus may already be present within a droplet, may contact the outside of a droplet and cause it to freeze, or may aid in ice formation directly from the vapor phase. Ice nuclei are considerably more rare than cloud condensation nuclei and are not as well understood.

Once initiated, ice crystals will generally grow rapidly because air that is saturated with respect to water is supersaturated with respect to ice; i.e., water vapor will condense on an ice surface more readily than on a liquid surface. The habit, or shape, of an ice crystal is hexagonal and may be platelike, columnlike, or dendritic (similar to the snowflakes cut from paper by children). Habit depends primarily on the temperature of an ice crystals formation. If an ice crystal grows large enough to fall through air of varying temperatures, its shape can become quite intricate. Ice crystals can also grow to large sizes by aggregation (clumping) with other types of ice crystals that are falling at different speeds. Snowflakes are formed in this way.

Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes another object. If a supercooled droplet collides with an ice crystal, it will attach itself to the crystal and freeze. Supercooled water that freezes immediately will sometimes trap air, forming opaque (rime) ice. Supercooled water that freezes slowly will form a more transparent substance called clear ice. As droplets continue to collide with ice, eventually the shape of the original crystal will be obscured beneath a dense coating of ice; this is how a hailstone is formed. Hailstones may even contain some liquid water in addition to ice. Thunderstorms are dramatic examples of vigorous mixed clouds that can produce high precipitation rates. The electrical charging of precipitation particles in thunderstorms can eventually cause lightning discharges.

Measurement of precipitation

Precipitation reaching the ground is measured in terms of precipitation rate or precipitation intensity. Precipitation intensity is the depth of precipitation reaching the ground per hour, while precipitation rate may be expressed for different time periods. Typical precipitation rates for the northeastern United States are 2-3 in (50-80 mm) per month, but in Hilo, Hawaii, 49.9 in (127 cm) of rain fell in March 1980. Average annual precipitation exceeds 80 in (200 cm) in many locations. Because snow is less compact than rain, the mass of snow in a certain depth may be equivalent to the mass of rain in only about one-tenth that depth (i.e., 1 in (2.5 cm) of rain contains as much water as about 10 in [25 cm] of snow). Certain characteristics of precipitation are also measured by radar and satellites.

Hydrologic cycle

Earth is unique in the solar system in that it contains water, which is necessary to sustain life as we know it. Water that falls to the ground as precipitation is critically important to the hydrologic cycle, the sequence of events that moves water from the atmosphere to Earths surface and back again. Some precipitation falls directly into the oceans, but precipitation that falls on land can be transported to the oceans through rivers or under-ground in aquifers. Water stored in this permeable rock can take thousands of years to reach the sea. Water is also contained in reservoirs such as lakes and the polar ice caps, but about 97% of Earths water is contained in the oceans. The suns energy heats and evaporates water from the ocean surface. On average, evaporation exceeds precipitation over the oceans, while precipitation exceeds evaporation over land masses. Horizontal air motions can transfer evaporated water to areas where clouds and precipitation subsequently form, completing the circle which can then begin again.

The distribution of precipitation is not uniform across Earths surface, and varies with time of day, season and year. The lifting and cooling that produces precipitation can be caused by solar heating of Earths surface, or by forced lifting of air over obstacles or when two different air masses converge. For these reasons, precipitation is generally heavy in the tropics and on the upwind side of tall mountain ranges. Precipitation over the oceans is heaviest at about 7°N latitude (the inter-tropical convergence zone), where the trade winds converge and large thunderstorms frequently occur. While summer is the wet season for most of Asia and northern Europe, winter is the wettest time of year for Mediterranean regions and western North America. Precipitation is frequently associated with large-scale low-pressure systems (cyclones) at mid-latitudes.

Human influences on precipitation

Precipitation is obviously important to humankind as a source of drinking water and for agriculture. It cleanses the air and maintains the levels of lakes, rivers, and oceans, which are sources of food and recreation. Interestingly, human activity may influence precipitation in a number of ways, some of which are intentional, and some of which are quite unintentional. These are discussed below.

Cloud seeding

The irregular and frequently unpredictable nature of precipitation has led to a number of direct attempts to either stimulate or hinder the precipitation process for the benefit of humans. In warm clouds, large hygroscopic particles have been deliberately introduced into clouds in order to increase droplet size and the likelihood of collision and coalescence to form raindrops. In cold clouds, ice nuclei have been introduced in small quantities in order to stimulate precipitation by encouraging the growth of large ice crystals; conversely, large concentrations of ice nuclei have been used to try to reduce numbers of supercooled droplets and thereby inhibit precipitation formation. Silver iodide, which has a crystalline structure similar to that of ice, is frequently used as an ice nucleus in these cloud seeding experiments. Although certain of these experiments have shown promising results, the exact conditions and extent over which cloud seeding works and whether apparent successes are statistically significant is still a matter of debate.

Acid rain

Acid rain is a phenomenon that occurs when acidic pollutants are incorporated into precipitation. It has been observed extensively in the eastern United States and northern Europe. Sulfur dioxide, a gas emitted by power plants and other industries, can be converted to acidic sulfate compounds within cloud droplets. In the atmosphere, it can also be directly converted to acidic particles, which can subsequently act as CCN or be collected by falling raindrops. About 70 megatons of sulfur is emitted as a result of human activity each year across the planet. (This is comparable to the amount emitted naturally.) Also, nitrogen oxides are emitted by motor vehicles, converted to nitric acid vapor, and incorporated into clouds in the atmosphere.

Acidity is measured in terms of pH, the negative logarithm of the hydrogen ion concentration; the lower the pH, the greater the acidity. Water exposed to atmospheric carbon dioxide is naturally slightly acidic, with a pH of about 5.6. The pH of rainwater in remote areas may be as low as about 5.0 due to the presence of natural sulfate compounds in the atmosphere. Additional sulfur and nitrogen containing acids introduced by anthropogenic (human-induced) activity can increase rainwater acidity to levels that are damaging to aquatic life. Recent reductions in emissions of sulfur dioxide in the United Kingdom have resulted in partial recovery of some affected lakes.

Greenhouse effect

Recent increases in anthropogenic emissions of trace gases (for example, carbon dioxide, methane, and chloroflourocarbons) have resulted in concern over the so-called greenhouse effect. These trace gases allow energy in the form of sunlight to reach Earths surface, but trap or absorb the infrared energy (heat) that is emitted by Earth. The heat absorbed by the atmosphere is partially re-radiated back to Earths surface, resulting in warming. Trends in the concentrations of these greenhouse gases have been used in climate models (computer simulations) to predict that the global average surface temperature of Earth will warm by 3.610.8°F(26°C) within the twenty-second century. For comparison, the difference in average surface temperature between the Ice Age 18,000 years ago and present day is about 9°F(5°C).

Greenhouse warming due to anthropogenic activity is predicted to have other associated consequences, including rising sea level and changes in cloud cover and precipitation patterns around the world. For example, a reduction in summertime precipitation in the Great Plains states is predicted by many models and could adversely affect crop production. Other regions may actually receive higher amounts of precipitation than they do currently. The level of uncertainty in these model simulations is high, however, due to approximations that are made. This is especially true of calculations related to aerosol particles and clouds. Also, the natural variability of the atmosphere makes verification of any current or future trends extremely difficult unless actual changes are quite large.

Effects of particulate pollution on cloud microphysics

As discussed above, gas-phase pollutants such as sulfur dioxide can be converted into water-soluble particles in the atmosphere. Many of these particles can then act as nuclei of cloud droplet formation. Increasing the number of CCN in the atmosphere is expected to change the characteristics of clouds. For example, ships emissions have been observed to cause an increase in the number of droplets in the marine stratus clouds above them. If a constant amount of liquid water is present in the cloud, the average droplet size will be smaller. Higher concentrations of smaller droplets reflect more sunlight, so if pollution-derived particles alter clouds over a large enough region, climate can be affected. Precipitation rates may also decrease, since droplets in these clouds are not likely to grow large enough to precipitate.

KEY TERMS

Aerosol particles Solid or liquid particles suspended in the air.

Cold cloud A cloud that exists, at least in part, at temperatures below 32°F(0°C).

Hailstone Precipitation that forms when super-cooled droplets collide with ice and freeze.

Mixed cloud A cloud that contains both liquid water and ice.

Supercooled Water than exists in a liquid state at temperatures below 32°F(0°C).

Virga Precipitation that evaporates before reaching the ground.

Warm cloud A cloud that exists entirely at temperatures warmer than 32°F(0°C).

See also Seasons; Thunderstorm; Weather modification.

Resources

BOOKS

Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Belmont, CA: Thomson/Brooks/Cole, 2007.

Douglas, Paul. Restless Skies: The Ultimate Weather Book. New York: Sterling Publishing Company, 2005.

Lutgens, Frederick K. The Atmosphere: An Introduction to Meteorology. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

McElroy, Michael B. The Atmospheric Environment: Effects of Human Activity. Princeton, NJ: Princeton University Press, 2002.

Cynthia Twohy Ragni

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Precipitation

Precipitation

Rain
Drizzle
Virga
Showers
Freezing rain
Snow
Snowflakes
Snow grains and snow pellets
Intensity of snowfall
Blizzards
Avalanches
Ice
Ice pellets
Hailstones
For More Information

Precipitation is defined as any form of water that originates in the clouds and falls toward the ground. By this definition, precipitation includes rain, snow, and ice. Each of the three main forms of precipitation can be broken down into specific categories, according to the temperature of the air layers through which the precipitation passes, the size of the individual water particles, and the intensity with which it falls.

Most precipitation (except in the tropics) originates in clouds as ice crystals. As an ice crystal descends through a cloud, it grows by collecting water vapor and droplets of supercooled water, which remains in the liquid state below the freezing point. In the process, the ice crystal takes on the shape of a snowflake, a lump of snow or ice, or in severe thunderstorms, hail. What happens next depends on the air temperature at various heights throughout the ice crystal's descent.

If the air temperature remains below freezing throughout the entire descent, the precipitation will reach the ground in the frozen state as snow. If the ice crystal passes through a layer of air above the freezing point of water, the ice crystal will melt and fall as rain. However, if the melting raindrop passes through a freezing layer, it will refreeze and reach the ground as ice pellets, or frozen raindrops. Finally, if the ice crystal melts, only to reenter freezing air just at ground level, it will strike the ground as freezing rain.

Rain

Each raindrop is made up of a million or so microscopic cloud droplets. The average raindrop measures 0.04 to 0.24 inches (0.10 to 0.61 centimeters) in diameter. Raindrops that are larger than average become unstable and tend to break apart into smaller raindrops. "Rain" is defined as liquid water that falls from the sky. A rain water drop must be larger than 0.02 inches (0.05 centimeters). Precipitation consisting of anything smaller than that is called drizzle.

The main sources of rain are thick clouds with high bases, namely nimbostratus and cumulonimbus clouds. Occasionally rain falls from a thick layer of altostratus or tall cumulus clouds.

Some raindrops form in warm clouds, clouds that are too warm for ice crystals to form. Although raindrops also collide with water droplets as they descend through a cloud, the collisions are less likely to result in coalescence (the process by which an ice crystal grows larger). Hence, water drops do not grow as large as their ice-crystal counterparts. Raindrops from warm clouds are usually less than 0.08 inches (0.2 centimeters) in diameter.

The exception to this rule are raindrops that form in towering cumulus or cumulonimbus clouds in the tropics. In those clouds, where strong updrafts (columns of air blowing upward, inside a vertical cloud) exist, a water drop may be blown from the bottom to the top of the cloud several times, growing larger each time, until it finally falls to the ground. Raindrops up to 0.32 inches (0.8 centimeters) in diameter have been found falling from such clouds in Hawaii.

Visibility in the air improves following a rainfall because precipitation has a cleansing effect on the air. Water droplets in the air form around condensation nuclei, tiny particles of dust and debris. When precipitation falls, it removes these particles from the air.

Drizzle

Drizzle is precipitation made up of drops that are between 0.008 inches (0.02 centimeters) and 0.02 inches (0.05 centimeters) in diameter. These drops are just barely large enough to overcome the upward force of air resistance. Once they do, they slowly drift downward, sometimes taking over an hour to travel from the cloud to the ground.

Drizzle is produced in two ways. It falls from stratus clouds, which are low and exist in shallow layers less than 1.5 miles (about 2.5 kilometers) thick. In stratus clouds, water drops have far less opportunity to grow by coalescence than they do in rain-producing clouds. Drizzle can also form as it begins as rain, descends through dry air, and partially evaporates. By the time the raindrops reach the ground, they have been reduced to the size of drizzle.

Weather report: The shape of a raindrop

Contrary to popular belief, raindrops are not tear-shaped or pear-shaped. Raindrops vary in shape between a sphere and a lump, depending on their size.

Small raindrops, less than 0.08 inches (0.2 centimeters) in diameter, are nearly spherical. Raindrops with diameters greater than 0.08 inches look saucer-shaped, flattened at the bottom and rounded on the top. They are wider than they are tall. Large raindrops have been said to resemble falling parachutes, mushroom caps, and hamburger buns.

The shape of a raindrop is caused by surface tension and air resistance. Surface tension is the attraction between water molecules at the surface. Surface tension forces molecules into the configuration with the smallest surface area, which is a sphere. In larger raindrops, this shape is distorted by the effect of air pressure (the pressure exerted by the weight of air over a given area of Earth's surface). Air pressure is felt most strongly on the bottom of the drop and most weakly on the sides of the drop. It pushes the bottom upward, flattening it, while allowing the sides to expand.

If a raindrop becomes larger than 0.25 inches (0.6 centimeters) in diameter, it will flatten out even more. At the same time it will become pinched on the top, into a bow-tie shape. The two halves of the bow tie will bulge and become more pronounced until the drop divides into two smaller spherical drops.

Drizzle may fall constantly, even for an entire day. The heaviest drizzle is produced where warm, moist air rises along the side of a mountain and forms mountain-wave clouds. Drizzle that falls from these clouds can produce 0.4 inches (1 centimeter) of water per day.

The drops in drizzle fall very close together, which reduces visibility. In a heavy drizzle, it may be possible to see only five-sixteenths of a mile (0.5 kilometer) ahead.

Virga

When humidity is very low, rain or snow may completely evaporate into the air during its descent. This creates streaks of falling water, called virga. Virga looks like dark fringes extending from the base of a cloud.

Virga often sets the stage for heavier precipitation. It does this by increasing the humidity of the air into which it evaporates. Thus, water or ice that falls may subsequently make it to the ground. Virga can also provide additional condensation nuclei for another cloud.

Showers

A shower is a spell of heavy, localized rainfall that occurs only in warm weather. Showers fall from towering cumuliform clouds, which are produced by strong convection currents (the circular movements of gas or liquid between hot and cold areas). Convection is the rising of pockets of warm air that occurs when Earth's surface is heated.

A shower occurs only while the shower-producing cumuliform cloud is overhead. It can last anywhere from two minutes to a half hour, depending on wind speed and the size of the cloud. In an area where a series of cumuliform clouds exist, several showers may occur, separated by dry, even sunny, periods.

The area being showered at any given time is no larger than 4 to 5 square miles (10 to 13 square kilometers). Rain, in contrast, can fall on an area larger than 100 square miles (260 square kilometers) at a time and can last all day.

Towering cumuliform clouds give rise to showers because they are able to generate large quantities of raindrops quickly. Ice crystals or water drops bounce between the top and bottom of the cloud numerous times, growing larger by coalescence on each trip. In most cases, by the time an ice crystal reaches the bottom of the cloud, the air is warm enough to cause the ice to melt.

When a water drop becomes too large, it breaks apart, forming smaller drops. These drops, in turn, get blown to the top of the cloud, and the process of coalescence is repeated. A chain reaction ensues, producing more and more drops. This reaction continues until the updrafts weaken or change direction. Then a sudden, heavy shower falls.

The heaviest showers are called cloudbursts. To qualify as a cloudburst, precipitation must fall at a rate of 4 inches (10 centimeters) or more per hour. The heaviest showers occur in the tropics, where the air is warm and moist, and powerful convection currents produce huge thunderstorm clouds.

Freezing rain

Freezing rain, as its name suggests, is rain that freezes on the ground. Freezing rain begins its journey to the surface as snow. As the snow descends, it encounters a layer of warm air and melts. Then, just above the ground, it travels through a shallow layer of subfreezing air. The raindrops don't have time to refreeze but remain in the liquid state at temperatures below freezing. In other words, they become supercooled.

When the supercooled liquid makes contact with a cold surface, it spreads out and then freezes. It forms a layer of clear, smooth ice called glaze. If drizzle becomes supercooled and then freezes on the ground, it is known as freezing drizzle. Freezing rain creates a beautiful, but hazardous, coating of ice on trees, power lines, and roads.

Freezing rain occurs most often in the winter, following a cold night in which the ground rapidly loses heat to the atmosphere through radiational cooling. As a result, an inversion is produced. An inversion exists when a layer of cold air is found next to the ground and a warmer layer of air lies above.

A heavy downpour of freezing rain is called an ice storm. While the layer of glaze deposited by freezing rain is usually less than an inch thick, it can be much thicker in ice storms. For instance, an ice storm in northern Idaho in January 1961 produced an 8-inch-thick (20-centimeter-thick) layer of glaze. It has been estimated that during a severe ice storm, a 50-foot-tall (15-meter-tall) evergreen tree with an average width of 20 feet (6 meters) may be loaded down with five tons of ice.

A key reference to: Measuring the intensity of rainfall

The intensity of precipitation is the amount of water that falls over a given period of time. For instance, a cloudburst is the most intense form of rainfall because it releases a large amount of water to the ground in a very short period of time. On the other hand, a steady rain may produce the same amount of water as a cloudburst, but since it falls over a longer period, it has a lower intensity.

The following are commonly accepted definitions for categories of rainfall, based on intensity:

  • Heavy rain: greater than 0.3 inches (0.75 centimeters) of water falls per hour. Heavy rain appears to fall in sheets and greatly reduces visibility.
  • Moderate rain: between 0.1 and 0.3 inches (0.25 and 0.76 centimeters) of water falls per hour. While the rain does not fall in sheets, it still falls too fast to see individual raindrops.
  • Light rain: less than 0.1 inches (0.25 centimeters) of water falls per hour. Individual raindrops can be seen.
  • Trace: rainfall is too light to measure.

The intensity of drizzle, which produces very small quantities of water, is measured in terms of visibility.

  • Heavy drizzle: visibility is restricted to 0.31 miles (0.5 kilometers).
  • Moderate drizzle: visibility is between 0.31 and 0.62 miles (0.5 and 1 kilometer).
  • Light drizzle: visibility is greater than 0.62 miles (1 kilometer).

In an ice storm, it is common for tree limbs and power and telephone lines to be knocked down. The ice is very difficult to drive on and causes traffic accidents. Over 85 percent of the deaths that occur in ice storms are traffic related.

One of the most severe ice storms on record struck a huge area, from Mississippi to New England, in January 1983. That storm resulted in an estimated 25 deaths and cut off power to more than 250,000 people.

Freezing rain is common in hilly or mountainous areas, where cold air sinks into the valleys. In the Appalachian mountains of Pennsylvania and West Virginia, freezing rain can fall on localized areas for long periods of time. The ice deposited by freezing rain usually melts within a few hours, although occasionally it can persist for days. The record for the longest-lasting glaze was set in 1969. In that year, ice remained on the trees for six weeks in Connecticut.

Snow

Snow is precipitation that is common during the winter in middle latitudes (the regions between 30° and 60°, north and south) and year-round on mountaintops. Its basic unit is the snowflake, which consists of many ice crystals joined to each other. Like raindrops, snow-flakes come in various shapes and sizes. Like rain, snow is categorized by the intensity with which it falls: from the lightest form, flurries, to the heaviest form, a blizzard.

In order to qualify as snow, precipitation must remain frozen when it reaches the ground. This does not mean, however, that it can snow only when the surface air temperature is at or below freezing.

Rather, snow can remain frozen at temperatures above freezing for distances up to 1,000 feet (300 meters) without melting. The exact air temperature at which snow turns to rain depends on the humidity of the air.

At the beginning of a snowfall, when temperatures are above freezing, snow may turn to rain as it falls. If air is dry, rain rapidly evaporates into it. The process of evaporation (in which water changes from a liquid to a gas) draws heat from the air, leaving the air cooler than before the evaporation began.

Weather report: Rainmaking

People throughout the ages have sought ways to bring rain to moisture-deprived areas. In the 1800s and early 1900s people tried, unsuccessfully, to produce rain by ringing church bells or firing cannons into the air. In the mid-1940s, scientists began testing an experimental method called cloud seeding. Cloud seeding involves injecting particles into a cloud, which act as freezing nuclei. Cloud droplets adhere to the particles and fall to the ground as precipitation.

A requirement of cloud seeding is that clouds are already present. Those clouds must be tall enough so that their upper portions extend into regions where temperatures are below freezing. In other words, they must be cold clouds. That is because the droplets that stick to the injected particles must be supercooled, meaning they exist in the liquid state at temperatures below freezing. Supercooled water droplets are found only in cold clouds.

The earliest cloud seeding experiments, performed in 1946 by atmospheric scientist Vincent Schaefer, involved dropping crushed dry ice (carbon dioxide) pellets into the top of a cloud from an airplane. Since dry ice is extremely cold (−108°F or −78°C), it cools the air around it, which produces more condensation. The dry ice pellets also act as freezing nuclei. In one experiment, Schaefer dropped three pounds of ground dry ice into an altocumulus cloud. Five minutes later, snow began falling from that cloud. The following year one of Schaefer's colleagues, Bernard Vonnegut, discovered that silver iodide makes a better cloud seeding agent than dry ice.

The 1950s saw a boom of rainmaking operations in drought-stricken areas around the world that experienced extended periods of abnormal dryness. These operations failed, however, to produce a significant increase in precipitation. Furthermore, they provoked the objections of many people who were concerned with the broader implications of manipulating the atmosphere. Would cloud seeding irreversibly alter the Earth's water cycle? Would it lead to uncontrollable flooding or prolonged droughts?

Despite these concerns, at the end of the 1950s a U.S. federal research program was launched on cloud seeding and other forms of weather modification. Government agencies and private companies conducted many experiments, with inconclusive results.

Cloud seeding was the center of controversy more than once in the 1970s. In 1972, a spate of cloud seeding was followed by a flash flood (a sudden, intense, localized flooding caused by persistent, heavy rainfall or the failure of a levee or dam) in Rapid City, South Dakota, in which more than two hundred people lost their lives. While the cloud seeding and the flash flood may or may not have been linked, the tragedy was enough to deter people from further rainmaking experiments there. Cloud seeding was also a questionable practice used by the U.S. military during the Vietnam War. After the flooding of the Ho Chi Minh Trail, which supposedly came about as a result of cloud seeding by the U.S. military, the U.S. Senate called the practice into question.

Does cloud seeding really work? More than sixty years after the first cloud-seeding experiment this question is still hotly debated. Some studies suggest that, under particular circumstances, cloud seeding can increase precipitation by 5 to 20 percent.

Some of the most impressive results have been obtained by seeding cumulus clouds. As droplets freeze to the injected particles, they release latent heat (the energy that is either absorbed by or released by a substance as it undergoes a phase change), which fuels the upward expansion of the clouds. When a cumulus cloud develops vertically, it lasts longer and is more likely to produce precipitation. Another way in which cloud seeding seems to be effective is by seeding winter clouds that are already producing precipitation. This practice has been shown to increase the amount of snow falling from those clouds.

On the flip side, by overseeding a cloud, it's possible to reduce precipitation. When too many freezing nuclei are present, they remain too small to fall to the ground—even after all available supercooled droplets have frozen onto the freezing nuclei. The moisture in the cloud will then evaporate. For this reason, overseeding is used at airports to dissipate thick fog (clouds that form near the ground).

The practice of cloud seeding continues to the present day. Current expectations of what cloud seeding can produce, however, are quite humble compared to the expectations of researchers in the 1940s and 1950s. In the United States, cloud seeding is performed mainly in California. There, the goal of cloud seeding is to slightly increase the amount of rainfall or snowfall produced by storms.

As snow continues to fall from the cloud, it encounters lower temperatures, although temperatures may still be above freezing. If the snow enters above-freezing temperatures, it begins to melt. Water from the edge of a snowflake rapidly evaporates into the air, further cooling the air, as well as cooling the snowflake.

If cooled below the freezing point by the evaporative process, the snowflake will reach the ground intact. If not, it will melt, and some of the water will evaporate, further lowering the air temperature. As long as air remains unsaturated (with less than 100 percent relative humidity, a measure of humidity as a percentage of the total moisture a given volume of air, at a particular temperature, can hold), the cooling process will continue. However, once air becomes saturated (has 100 percent relative humidity), no net evaporation will occur, and cooling will cease.

Snowflakes

The first person to study the intricate detail and uniqueness of snowflakes extensively was an American farmer named Wilson Bentley (1865–1931). Beginning in 1880, when he was just fifteen years old, Bentley placed snowflakes under a microscope and photographed them. He continued this work for fifty years, making thousands of photographs. In 1931 he published a book with W. J. Humphreys, entitled Snow Crystals, which contained more than 2,300 photos of snow and frost.

One thing that all snowflakes have in common is a hexagonal (six-sided) configuration. The structure of the ice crystals that make up a snowflake is also hexagonal. In fact, this basic shape can be traced back to water molecules. Because of the electrical attraction between water molecules, they take a hexagonal shape when they freeze.

A snowflake begins its existence as an ice crystal within a cold cloud. As it bounces between the bottom and top of the cloud, it grows by coalescence with supercooled water drops or by deposition, the freezing of water vapor molecules directly onto the ice crystal. As the ice crystal grows, it bonds with other ice crystals and assumes the shape of a snowflake (also called a snow crystal). When the snowflake becomes heavy enough, it descends to the surface.

Weather report: Ice and aircraft

Freezing rain and supercooled water droplets within clouds pose major hazards to aircraft. As an aircraft travels through clouds where the temperature is between 10 and 32°F (−12 and 0°C), it encounters a mixture of ice crystals and supercooled droplets. Where supercooled raindrops or large droplets in clouds come in contact with an aircraft, they spread out and form an even coating of ice called glaze. Where the small droplets strike an aircraft, they freeze immediately without spreading, trapping air bubbles. The second type of icy coating, which appears white and weighs less than glaze, is called rime.

A layer of glaze, and to a lesser extent a layer of rime, affects an aircraft in several ways. First, it makes the plane heavy—sometimes so heavy that it is literally pulled from the sky. Second, if ice forms on the plane's wing or fuselage, it provides wind resistance and alters the plane's aerodynamics. If the engine's air intake opening is iced over, it can lead to a loss of power. Ice can also cause the failure of brakes, landing gear, or instruments.

Ice poses the greatest danger to small, single- or twin-engine planes. Ice affects jet airliners to a lesser extent since those planes spend most of their time above the clouds. Nonetheless, airline pilots are given warnings of, and strive to avoid, clouds that may contain supercooled droplets. As an added precaution, the wings of an aircraft are usually de-iced, meaning they are sprayed with a type of antifreeze before take-off.

Snowflakes can exist in flat, platelike forms; long, six-sided columns; or needles that are two hundred times longer than they are wide. They may also form starry shapes, called sector plates. When a sector plate accumulates moisture, it may develop feathery branches on its arms. In this way, the most distinctive and most common snowflake, the dendrite, is formed. As dendrites travel through the cloud, they may combine with other dendrites, forming a wide array of complex patterns.

The shape of a snowflake depends upon the air temperature within the cloud where it is formed.

  1. Below −8°F (−22°C), snowflakes are hollow columns.
  2. Between −8 and 3°F (−22 and −16°C), they are sector plates.
  3. Between 3 and 10°F (−16 and −12°C), they are dendrites.
  4. Between 10 and 14°F (−12 and −10°C), they are sector plates.
  5. Between 14 and 21°F (−10 and −6°C), they are hollow columns.
  6. Between 21 and 25°F (−6 and −4°C), they are needles.
  7. Over 25°F (−4°C), they are thin hexagonal plates.

Why are dendrites the most common form of snowflake, given that they only form within a narrow range of temperatures (10 and 14°F or −12 and −10°C)? The reason is that dendrites form more rapidly than do other snowflake types.

In the dendrite-forming temperature range, the difference in vapor pressure (the pressure exerted by a vapor when it is in equilibrium with its liquid or solid) between water droplets and ice crystals is greatest. Vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its liquid or solid. Vapor pressure is greater over the surface of a water droplet than it is over the surface of an ice crystal. Similar to air, water molecules migrate from an area of high pressure to an area where pressure is lower. Thus, when a water droplet comes in contact with an ice crystal, water molecules leave the water droplet and freeze onto the ice crystal. The greater the difference in vapor pressure between water and ice, the more rapidly ice crystal growth occurs.

Question: Why do we salt icy roads?

Salt is applied to snowy, icy roads because it melts ice and prevents the water from refreezing. Compared to other materials that are used to combat slippery roadways, such as sand and cinders, salt is relatively cheap and easy to apply. For these reasons, salt has been the road de-icing agent of choice since the 1960s.

The chemical composition of salt used on roads is sodium chloride (NaCl). When it comes in contact with water molecules, NaCl breaks down into one positively charged sodium ion (NA+2) and two negatively charged chloride ions (Cl–). A water molecule consists of one oxygen atom (O−2) and two hydrogen atoms (H+). The positively charged hydrogen atoms are drawn to the negatively charged chloride atoms. Negatively charged oxygen atoms are drawn to the sodium atoms.

Sodium chloride thus causes the components of individual water molecules to disassociate. It bonds with the hydrogen and oxygen atoms so that hydrogen and oxygen are not free to recombine into water. The sodium and chloride ions also draw water molecules away from one another. Salt both prevents liquid water molecules from forming ice crystals and breaks up existing ice crystals.

Sodium chloride lowers the freezing point of water from 32°F (0°C) to 20°F (−7°C). At temperatures below 20°F, salt is no longer effective at melting ice.

Whereas salt provides an efficient means of melting snow and ice on roads, and has greatly contributed to highway safety, it also has a downside. Salt is bad for the environment. It kills vegetation along the side of the road and can seep down into wells, making the water undrinkable. Salt also causes vehicles to rust and bridges to corrode. For these reasons, salt is only applied in the minimum quantities necessary to get the job done.

The size of a snowflake depends upon the temperature of the air as the snowflake descends. When a snowflake falls through air in which the temperature is above freezing, it melts around the edges. A film of water forms that acts like glue. It causes snowflakes that come in contact with one another to stick together, producing large, soggy snowflakes, 2 to 4 inches (5 to 10 centimeters) or larger in diameter. These snowflakes stick to surfaces and are heavy to shovel.

In contrast, snowflakes that fall through very cold dry air do not readily stick together. They are small and powdery when they hit the ground. This snow makes for ideal skiing conditions.

Snow grains and snow pellets

Snow grains are the frozen equivalent of drizzle. They are small, soft white grains of ice that form within stratus clouds and fall to the ground in only small amounts. Snow grains are elongated and generally have diameters less than 0.04 inches (0.1 centimeters). Because they are so light, they fall very slowly and land gently, without bouncing or shattering.

In contrast to snow grains, snow pellets fall rapidly and bounce high off the ground. These white pieces of icy matter, also called graupel or soft hail, measure between 0.08 and 0.2 inches (0.2 and 0.5 centimeters) in diameter. Snow pellets fall in showers and feel brittle and crunchy underfoot.

Snow pellets form within towering cumuliform clouds, where the atmosphere is very unstable. This instability occurs when air temperature drops rapidly with height. The top of the cloud, where temperatures are lowest, is inhabited mostly by ice crystals. The ice crystals grow by the deposition of water vapor onto them and take on the shape of snowflakes.

As a snowflake travels downward into the warmer, middle region of the cloud, it encounters supercooled water droplets. In what is known as riming, the droplets freeze to the snowflake, trapping numerous air pockets in the process. If riming occurs to a great enough extent, the snowflake will be transformed into a lumpy, white pellet of snow called graupel. It is in this form that precipitation reaches the surface.

Intensity of snowfall

The lightest form of snowfall is flurries. Flurries are brief and intermittent and originate in cumuliform clouds. While they produce very little accumulation, flurries may interfere with visibility.

A heavier and more persistent snowfall, which most people think of as snow, comes from nimbostratus and altostratus clouds. This snowfall may continue steadily for several hours.

A brief but heavy snow shower is called a snow squall. Snow squalls, like flurries usually originate in cumuliform clouds. Snow squalls can be compared in intensity to summer rain showers and are accompanied by strong surface winds.

Heavy snow is defined as that which reduces visibility to 0.3 miles (0.5 kilometers). Heavy snow, on average, yields 4 inches (10 centimeters) or more in a twelve-hour period or 6 inches (15 centimeters) or more in a twenty-four hour period. However, the amount of snow accumulation deemed heavy varies from one geographic area to another. For instance, in places where accumulations of 4 inches during a twelve-hour period are common, snow may not be considered heavy until more than 6 inches have accumulated during that period. On the other hand, where any accumulation of snowfall is rare, an accumulation of 2 to 3 inches (5 to 8 centimeters) in a twelve-hour period may be considered heavy.

Snow can also be classified by how it behaves on the surface. For instance, drifting snow is loose snow that has been swept by strong winds into large piles, or drifts. Blowing snow is snow that has been lifted off the surface by the wind and blown about in the air. Blowing snow may reduce visibility in a manner similar to that which occurs in a heavy snowfall. A ground blizzard is the condition that results when snow continues to drift and blow after a snowfall has ended.

Blizzards

A blizzard is the most severe type of winter storm. It is characterized by strong winds, large quantities of snow, and low temperatures. The National Weather Service defines a blizzard as a snowstorm with winds of 35 mph (56 kph) or greater. The temperature is generally 20°F (−7°C) or lower. The falling and blowing of fine powdery snow greatly reduces visibility, often to less than a quarter of a mile and sometimes to just a few yards.

When a blizzard strikes, it can bring traffic to a standstill, strand motorists, and shut down entire cities. Prolonged exposure to a blizzard can cause frostbite (the freezing of the skin), hypothermia (a condition characterized by a drop in core body temperature from the normal 98.6°F to below 95°F), and even death. Some people have actually suffocated to death during blizzards by choking on fine, powdery snow.

A severe blizzard is a blizzard in which wind speeds exceed 45 mph (72 kph), the snowfall is heavy, and the temperature is no higher than 10°F (−12°C). When falling, drifting, and blowing snow reduce visibility to almost zero, the condition is called a whiteout. Everything appears white, making the ground and sky indistinguishable. People stranded in such conditions can easily become disoriented and lose their way.

Can two snowflakes be alike?

To answer this question, a definition of "alike" is required. "Alike" can mean that two snow-flakes are the same, molecule for molecule, throughout. By this definition, the answer is no, two snowflakes cannot be alike. First of all, a snowflake contains over 180 billion water molecules. These molecules come together under many different conditions, making it all but impossible for any two snowflakes to have an identical configuration. In addition, water molecules are constantly freezing to and evaporating from snowflakes, meaning that snowflakes are constantly changing at the molecular level.

An alternate definition of "alike" is "identical in appearance." By this definition, the answer is yes, two snowflakes can be alike. This fact was discovered in 1989 by Nancy Knight, a cloud physicist with the National Center for Atmospheric Research. Knight collected snow samples while on board a research aircraft at a height of 20,000 feet (6 kilometers) over Wausau, Wisconsin. She discovered two hollow-column snowflakes, both 250 microns long and 170 microns wide (a micron is one-millionth of a meter—by way of comparison, a human hair is about 100 microns in diameter), proving that snowflakes can be identical in appearance. To this day, however, there is no record of identical dendrites.

Avalanches

An avalanche is the cascading of at least 100,000 tons of snow down a steep slope. It occurs when stress is placed on a weak layer of snow. For an avalanche to occur, snow on the ground must be layered in such a way that it is structurally unstable. For instance, a loose layer of snow may be sandwiched between two more compact layers.

Every time new snow falls, it places additional weight and pressure on the existing snow. At some point, this pressure may give rise to a fracture (break) across the blanket of snow, down to the weakest layer. As soon as any additional stress is added, a slab of snow breaks off and goes hurtling down the slope, The additional stress may take the form of more snow, a strong gust of wind, the weight of a skier, or even a loud noise.

Weather report: Lake-effect snow

Lake-effect snow is the name given to the heavy snowfalls that occur along the shorelines of the Great Lakes. The process that gives rise to lake-effect snow begins when dry, polar air masses, informally called Alberta Clippers, sweep down from Canada. As this air travels across the Great Lakes, evaporation in the form of steam fog raises the humidity of the air considerably. Clouds form and deposit heavy snowfall on the land downwind of the Great Lakes.

In January 1959, 51 inches (130 centimeters) of lake-effect snow fell during a sixteen-hour period on Bennetts Bridge, New York, on the southern shore of Lake Ontario. Lake-effect snow fell on Buffalo, New York, for forty straight days during the winter of 1976–77.

Avalanches are extremely destructive. They bury everything in their path, even cities, in a matter of seconds. The largest avalanches occur in the Andes, the Himalayas, and the mountains of Alaska. However it is in the Alps, where valley regions are heavily populated, that avalanches pose the greatest danger to humans.

A key reference to: Winter storm alerts and safety procedures

The National Weather Service issues winter storm alerts whenever snowfall is anticipated to be heavy enough to create dangerous travel conditions. There are four different types of these alerts based on the seriousness of the storm, defined as follows:

  • A winter weather advisory states that snow, sleet, freezing rain, or high winds may be on the way. It advises people to exercise caution when traveling.
  • A winter storm watch states that at least 6 inches of snow and an ice storm may be on the way. It advises people to limit their travels and to exercise great caution if they must venture onto the roads.
  • A winter storm warning states that a storm, including heavy snow and possibly ice, has already begun or will soon begin. It advises people not to travel, except in an emergency.
  • A blizzard warning states that a blizzard—including heavy snow, low temperatures, and winds of at least 35 mph (56 kph)—is on the way. The combination of heavy snowfall and low clouds make it appear that the ground and sky are a continuous white sheet (called a whiteout), making travel nearly impossible. This warning advises people to remain indoors.

Safety procedures:

If you live in an area affected by winter storms, it is wise to take the following precautions at the start of the season:

  • Store extra blankets and warm clothing and boots at home for every member of the family.
  • Put together a supplies kit for your home containing first aid materials, a battery-powered flashlight, a battery-powered radio, extra batteries, nonperishable food, a nonelectric can opener, and bottled water.
  • Store a similar supplies kit, plus the following equipment, in the trunk of your car: a shovel, a bag of sand, tire chains, jumper cables, and a piece of brightly colored cloth to tie to your antenna.
  • Keep your car's gas tank full to prevent the fuel line from freezing.

If you must go outside during a winter storm, follow these rules:

  • Wear several layers of lightweight clothing, gloves, a hat, and a scarf covering your mouth.
  • Walk carefully over icy ground.
  • When shoveling, take frequent breaks to avoid overexertion.
  • If you must drive, inform someone of your route, destination, and expected time of arrival.

If you get stranded in your car during a winter storm:

  • Stay with your car. Tie the brightly colored cloth to your antenna so rescuers can spot you. Don'tattempt to walk away. It's easy to become disoriented and lose your way in a snowstorm.
  • Only start the car and turn on the heater for ten minutes each hour. When the car is running, leave on the inside light so you can be spotted. When the car is not running, periodically check the tailpipe and clear it of snow, if necessary. If your tailpipe is blocked, dangerous fumes can back up into the car.
  • Move your arms and legs continuously to stay warm and maintain your blood circulation.
  • Let in fresh air by slightly opening the window that's opposite the direction of the blowing wind.

In the United States, between twelve hundred and eighteen hundred avalanches are reported each year. Most of these occur in the western states. When taking into account avalanches that go unnoticed or unreported, the actual number is much higher.

Colorado has the highest death rate caused by avalanches, with six to eight fatalities a year. Most of the people killed are skiers or snow-mobilers. The best way to stay out of an avalanche's path is to avoid snow-covered slopes at angles steeper than 30 degrees.

Ice

Ice pellets and hailstones are the two forms of precipitation that fall to the ground as hard, mostly transparent pieces of ice. Ice pellets and hailstones have little else in common besides their composition.

Ice pellets

Ice pellets are frozen raindrops. They are formed by precipitation that passes first through a warm layer of air and melts, then reenters a layer of freezing air and refreezes. The precipitation reaches the ground as tiny pellets of ice.

Ice pellets differ from freezing rain in the depth of the layer of freezing air through which they pass. Ice pellets encounter the freezing air at a higher elevation than does freezing rain. Thus, ice pellets have time to freeze before they hit the ground, while freezing rain freezes only on contact with the cold ground.

In the United States, ice pellets are also referred to as sleet. Sleet is a confusing term, however, since this word is used in Australia and Great Britain to refer to a mixture of rain and wet snow. Even in the United States, sleet is often used by the news media to describe slushy precipitation. For this reason, this text will use the term "ice pellets" instead of "sleet."

Weather report: Snow rollers

On rare occasions, Mother Nature gives us a hand in building a snowman by creating snow rollers. A snow roller is a lumpy, spherical or cylindrical mass of snow, generally less than 1 foot in diameter. It is created only under a very particular set of conditions. First, there must be a layer of smooth, hard, crusty old snow on the ground. Then, a light layer of new snow falls on top of the old snow. Finally, a strong warm wind blows in, rapidly raising the temperature.

This wind literally lifts up a patch of snow and rolls it along the surface snow, which is warm and sticky. This process continues until the accumulated snow becomes too heavy to be rolled any farther.

One place where snow rollers have been witnessed is in Boulder, Colorado. They are created by the strong, warm Chinook winds, or dry, warm winds that blow down the eastern slopes of the Rocky Mountains.

Ice pellets measure only 0.2 inches (0.5 centimeters) in diameter, are irregular in shape, and bounce when they hit a surface. Ice pellets can also be identified by the ping sound they make when they strike a glass or metal surface. An accumulation of ice pellets can create hazardous driving and walking conditions.

Hailstones

Hailstones are a larger and potentially much more destructive form of frozen precipitation than ice pellets. They have either a smooth or jagged surface and are either totally or partially transparent. While most hailstones are pea-sized, they may reach the size of softballs.

Large hailstones have been responsible for destroying crops, breaking windows, and denting cars. They have also caused numerous human and animal deaths. The largest single hailstone on record was about the size of a cantaloupe. It measured 7 inches (18 centimeters) in diameter and 18.75 inches (48 centimeters) in circumference, with a weight of just under 1 pound (.45 kilograms). It fell on Aurora, Nebraska, on June 22, 2003. The heaviest hailstone on record in the United States measured 5.5 inches (14 centimeters) in diameter and 17 inches (43 centimeters) in circumference, with a weight of 1.7 pounds (.77 kilograms). It fell on Coffeyville, Kansas, on September 3, 1970.

WORDS TO KNOW

accretion:
the process by which a hailstone grows larger, by gradually accumulating cloud droplets as it travels through a cloud.
accretion:
the process by which an ice crystal grows larger. The ice crystal collides and sticks together with water droplets as the ice crystal travels down through a cloud.
condensation:
the process by which water changes from a gas to a liquid.
condensation nucleus:
a tiny solid particle around which condensation of water vapor occurs.
convection:
the upward motion of an air mass or air parcel that has been heated.
deposition:
the process by which water changes directly from a gas to a solid, without first going through the liquid phase.
downdraft:
a downward blast of air from a thunderstorm cloud felt at the surface as a cool gust.
drizzle:
precipitation formed by raindrops between 0.008 inches and 0.02 inches in diameter.
freezing nuclei:
a tiny particle of ice or other solid onto which supercooled water droplets can freeze.
supercooled water:
water that remains in the liquid state below the freezing point.
updraft:
a column of air blowing upward inside a vertical cloud.

Hailstones are formed within cumulonimbus clouds during intense thunderstorms. A hailstone forms around a small particle, called an embryo. Objects that can act as embryos include ice crystals, frozen raindrops, graupel, dirt, or even insects. There have also been reports of larger organisms, such as frogs, being swept up in a tornado (a rapidly spinning column of air that extends from a thunderstorm cloud to the ground) and returning to Earth with a hailstone formed around them.

As an embryo travels through a cloud, it is coated by cloud droplets and grows larger by accretion (gradual accumulation). On reaching the bottom of the cloud, the developing hailstone is blown back up to the top by powerful updrafts. It repeats its journey down and up through the cloud many times. In the case of very strong updrafts, this process may last several minutes. When the hailstone becomes heavy enough to overcome the force of the updraft, it falls to the ground.

Thunderstorms, and hence hailstones, are warm-weather phenomena. As a hailstone descends toward the surface and encounters warm air, it begins to melt. Small hailstones may melt completely in the air. Hailstones that are large enough, however, melt only partially and reach the ground in the frozen state. In the tropics, where the air is very warm, hail always melts before reaching the ground, turning to rain.

When sliced in half, a hailstone resembles an onion, with a pattern of concentric rings. The number of rings is equal to the number of trips the hailstone made through the cloud. Up to twenty-five rings have been counted in large hailstones.

Layers of a hailstone alternate between clear ice and white ice, called rime. The clear layers are formed when the hailstone is in warmer air, in the lower portion of the cloud. There, supercooled water droplets are plentiful. They form a layer of water around the hailstone, which slowly freezes when the hailstone returns to the colder, upper portion of the cloud.

Weather report: Hail alley

Hail alley is the region of North America where hail frequently damages crops. It covers a north-south belt from Alberta, Canada, to Texas. It extends westward to the Rockies and eastward to the Mississippi River. Hailstorms occur with the greatest frequency in the Great Plains states. There, hail falls accompany 10 percent of all thunderstorms.

A hailstorm can flatten an entire field in minutes. Hail damage to crops in the United States alone tops $700 million per year. To guard against the chance that a single storm can wipe out an entire year's earnings, farmers in hail alley spend large sums of money on hail insurance. Illinois farmers top the list of insurance buyers, purchasing over $600 million worth of liability coverage annually.

The milky white layers are formed in the upper, freezing portion of the cloud, where supercooled droplets are scarcer and freeze directly onto the hailstone by the process of riming. In a manner similar to the process by which a graupel is formed, the droplets trap air bubbles when they freeze to the hailstone.

The accumulation of hailstones during a thunderstorm can be considerable. One of the largest hail falls on record reached a depth of 18 inches (46 centimeters) and occurred in Selden, Kansas, in June 1959. Occasionally, snowplows must be taken out of summer storage to clear roads of hailstones. This was the case in September 1988, in Milwaukee, Wisconsin, when hailstone drifts reached 18 inches. In August 1980, snowplows were required to remove hailstone drifts 6 feet (2 meters) deep in Orient, Iowa.

The hailstorms that have caused the greatest toll in terms of human life have occurred in Asia. In 1888, in northern India, hail killed 246 people. In 1932, in southeast China, a hailstorm claimed about two hundred lives and injured thousands. In 1986, in Bangladesh, a storm that produced some very unusual hailstones weighing more than 2 pounds (1 kilogram) each killed ninety-two people. In the United States, only two deaths have been attributed to hail in the last century.

If you are caught in a thunderstorm, watch for these warning signs of hail: a green tinge develops at the base of the cloud or the rain begins to take on a whitish color. If you witness either of these signs, it is time to collect your family and pets and move indoors.

[See AlsoClouds; Forecasting; Weather: An Introduction ]

For More Information

BOOKS

Ackerman, Steven, and John A. Knox. Meteorology: Understanding the Atmposphere. 2nd ed. Pacific Grove, CA: Thomson, Brooks, and Cole, 2006.

Ahrens, C. Donald. Meteorology Today. 8th ed. Belmont, CA: Thomson, Brooks, and Cole, 2006.

DeMillo, Rob. How Weather Works. Emeryville, CA: Ziffe-Davis Press, 1994.

Lutgens, Frederick, and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology. 10th ed. Upper Saddle River, NJ: Prentice Hall, 2007.

Wood, Richard A., ed.Weather Almanac. 11th ed. Detroit: Gale, 2004.

WEB SITES

CNN. Weather. 〈http://www.cnn.com/WEATHER/〉 (accessed August 15, 2006).

National Oceanic and Atmospheric Administration (NOAA). Storm Prediction Center. 〈http://www.spc.noaa.gov/〉 (accessed August 15, 2006).

Weather Channel. Storm Watch. 〈http://www.spc.noaa.gov/〉 (accessed August 15, 2006).

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Precipitation

Precipitation

Introduction

Falling rain or snow, known as precipitation, is an essential part of the hydrological cycle. It replenishes water supplies for plants, agriculture, and other human uses. Precipitation levels vary widely from place to place and help to shape local weather and overall climate. There are some very dry places on Earth, such as Antarctica, and also locations such as rain forests where there may be 100 in (250 cm) or more of rain every year. Drought, which is a reduction in the usual level of precipitation, can spell disaster by causing crops to fail.

Precipitation is pure compared to seawater, which contains mineral salts. However, it may react with certain gases (such as sulfur dioxide, or SO2, which is emitted from power stations). These reactions can form acid precipitation or acid rain, which can fall far from the source of pollution. This kind of acidic pollution can damage buildings, lakes and rivers, and trees.

Historical Background and Scientific Foundations

In the hydrological cycle, also known as the water cycle, liquid water evaporates to form water vapor in the atmosphere. The liquid water comes from bodies of water such as oceans and lakes, as well as from respiration and plant transpiration. Evaporation is the escape of the more energetic water molecules from the surface of water, leaving salts and other materials behind. Therefore, water vapor is purer than the water that is its source.

The amount of water vapor held in the air is known as humidity and it increases with temperature. A useful measure is relative humidity, which is humidity compared to the maximum amount of water vapor the air could hold at that temperature. Saturation point occurs when the air is holding as much water vapor as possible at a given temperature. Beyond this it will undergo condensation, where vapor turns into liquid or solid droplets of water. Condensation occurs around tiny, often invisible, particles of dust, ash, spores, or smoke called nuclei. Then the droplets accumulate, forming clouds. Air currents can keep cloud particles suspended in the atmosphere when they are small. However, they tend to increase in size and at some point gravity forces them to fall to Earth as precipitation. Many clouds do not form precipitation because their droplets are too small and continue to be supported in the atmosphere by air currents.

There are various kinds of precipitation. Rain refers to liquid droplets of a diameter between 0.02 and 0.2 in (0.05 and 0.5 cm). Smaller droplets are known as drizzle. Sleet is transparent pellets of frozen water that start off as rain but freeze as they fall through colder air. Snow forms when water vapor condenses as hexagonal solid particles, while hail is ice pellets more than 0.2 in (0.5 cm) in diameter.

Levels of precipitation vary widely around the world. Global air circulation patterns are very influential in shaping the amount of precipitation occurring in a region. Generally, where there is high pressure, there is low precipitation. This means that there tends to be low precipitation between latitude 20° and 40° North and South, but high precipitation between latitude 40° and 60° North and South, as well as near the equator. There is also more precipitation near the oceans and on the windward side of mountains, where air pressure is lower. Conversely, there tends to be less precipitation on the other, leeward, side of a mountain where air pressure rises.

Impacts and Issues

Precipitation is essential for replenishing water resources on land. But it can be damaging. Excessive rainfall can cause flooding that destroys crops and buildings in addition to putting lives at risk. Persistent rainfall can cause soil erosion and hail can devastate crops.

Sulfur (S) emissions from power stations running on coal can react with water vapor to create a dilute form of sulfuric acid known as acid precipitation or acid rain. This form of pollution was discovered by the Scottish chemist Robert Angus Smith in the mid-nineteenth century. Acid rain caused rapid forest decline at high elevations in many parts of Europe and North America in the 1980s. It also caused many lakes in Sweden to become so acidic that many fish species could no longer live there. Buildings and statues in cities around the world, from India’s Taj Mahal to Michaelangelo’s statue of David in Florence, are also under threat from acid rain. However, legislation against air pollution in recent years has started to limit the damage caused by this form of precipitation.

See Also Drought; Fossil Fuel Combustion Impacts; Water Resources

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

WORDS TO KNOW

CLOUD: A patch of condensed water or ice droplets.

CONDENSATION: The coalescence of water molecules from the vapor to the liquid or solid phase.

RELATIVE HUMIDITY: The amount of water vapor in the air compared to the maximum amount it could hold at that temperature.

SATURATION POINT: The maximum concentration of water vapor that the air can hold at a given temperature.

Kaufmann Robert, and Cutler Cleveland. Environmental Science. New York: McGraw-Hill, 2007.

Web Sites

PhysicalGeography.net. “Precipitation and Fog.” http://www.physicalgeography.net/fundamentals/8f.html (accessed April 16, 2008).

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Precipitation

Precipitation

In meteorology, precipitation is water in either solid or liquid form that falls in Earth's atmosphere. Major forms of precipitation include rain, snow, and hail. When air is lifted in the atmosphere, it expands and cools. Cool air cannot hold as much water in vapor form as warm air, and the condensation of vapor into droplets or ice crystals may eventually occur. If these droplets or crystals continue to grow to large sizes, they will eventually be heavy enough to fall to Earth's surface.


Types of precipitation

Precipitation in liquid form includes drizzle and raindrops. Raindrops are on the order of a millimeter (one thousandth of a meter) in radius, while drizzle drops are approximately a tenth of this size. Important solid forms of precipitation include snowflakes and hailstones. Snowflakes are formed by aggregation of solid ice crystals within a cloud, while hailstones involve supercooled water droplets and ice pellets. They are denser and more spherical than snowflakes. Other forms of solid precipitation include graupel and sleet (ice pellets). Solid precipitation may reach the earth's surface as rain if it melts as it falls. Virga is precipitation that evaporates before reaching the ground.


Formation of precipitation

Precipitation forms differently depending on whether it is generated by warm or cold clouds . Warm clouds are defined as those that do not extend to levels where temperatures are below 32°F (0°C), while cold clouds exist at least in part at temperatures below 32°F (0°C). Temperature decreases with height in the lower atmosphere at a moist adiabatic rate of about 3.3°F per 3,281 ft (1.8°C per 1,000 m), on average. High clouds, such as cirrus, are therefore colder and more likely to contain ice. As discussed below, however, temperature is not the only important factor in the formation of precipitation.


Precipitation formation in warm clouds

Even the cleanest air contains aerosol particles (solid or liquid particles suspended in the air). Some of these particles are called cloud condensation nuclei, or CCN, because they provide favorable sites on which water vapor can condense. Air is defined to be fully saturated, or have a relative humidity of 100%, when there is no net transfer of vapor molecules between the air and a plane (flat) surface of water at the same temperature. As air cools, its relative humidity will rise to 100% or more, and molecules of water vapor will bond together, or condense, on particles suspended in the atmosphere. Condensation will preferentially occur on particles that contain water soluble (hygroscopic) material. Types of particles that commonly act as CCN include sea-salt and particles containing sulfate or nitrate ions; they are typically about 0.0000039 in (0.0001 mm) in radius. If relative humidity remains sufficiently high, CCN will grow into cloud droplets 0.00039 in (0.01 mm) or more in size. Further growth to precipitation size in warm clouds occurs as larger cloud droplets collide and coalesce (merge) with smaller ones.


Precipitation formation in cold clouds

Although large quantities of liquid water will freeze as the temperature drops below 32°F (0°C), cloud droplets sometimes are "supercooled;" that is, they may exist in liquid form at lower temperatures down to about -40°F (-40°C). At temperatures below -40°F (-40°C), even very small droplets freeze readily, but at intermediate temperatures (between -40 and 32°F or -40 and 0°C), particles called ice nuclei initiate the freezing of droplets. An ice nucleus may already be present within a droplet, may contact the outside of a droplet and cause it to freeze, or may aid in ice formation directly from the vapor phase. Ice nuclei are considerably more rare than cloud condensation nuclei and are not as well understood.

Once initiated, ice crystals will generally grow rapidly because air that is saturated with respect to water is supersaturated with respect to ice; i.e., water vapor will condense on an ice surface more readily than on a liquid surface. The habit, or shape, of an ice crystal is hexagonal and may be plate-like, column-like, or dendritic (similar to the snowflakes cut from paper by children). Habit depends primarily on the temperature of an ice crystal's formation. If an ice crystal grows large enough to fall through air of varying temperatures, its shape can become quite intricate. Ice crystals can also grow to large sizes by aggregation (clumping) with other types of ice crystals that are falling at different speeds. Snowflakes are formed in this way.

Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes another object. If a supercooled droplet collides with an ice crystal, it will attach itself to the crystal and freeze. Supercooled water that freezes immediately will sometimes trap air, forming opaque (rime) ice. Supercooled water that freezes slowly will form a more transparent substance called clear ice. As droplets continue to collide with ice, eventually the shape of the original crystal will be obscured beneath a dense coating of ice; this is how a hailstone is formed. Hailstones may even contain some liquid water in addition to ice. Thunderstorms

are dramatic examples of vigorous mixed clouds that can produce high precipitation rates. The electrical charging of precipitation particles in thunderstorms can eventually cause lightning discharges.


Measurement of precipitation

Precipitation reaching the ground is measured in terms of precipitation rate or precipitation intensity. Precipitation intensity is the depth of precipitation reaching the ground per hour, while precipitation rate may be expressed for different time periods. Typical precipitation rates for the northeastern United States are 2-3 in (50-80 mm) per month, but in Hilo, Hawaii, 49.9 in (127 cm) of rain fell in March 1980. Average annual precipitation exceeds 80 in (200 cm) in many locations. Because snow is less compact than rain, the mass of snow in a certain depth may be equivalent to the mass of rain in only about one-tenth that depth (i.e., 1 in [2.5 cm] of rain contains as much water as about 10 in [25 cm] of snow). Certain characteristics of precipitation are also measured by radar and satellites.


Hydrologic cycle

Earth is unique in our solar system in that it contains water, which is necessary to sustain life as we know it. Water that falls to the ground as precipitation is critically important to the hydrologic cycle , the sequence of events that moves water from the atmosphere to the earth's surface and back again. Some precipitation falls directly into the oceans, but precipitation that falls on land can be transported to the oceans through rivers or underground in aquifers. Water stored in this permeable rock can take thousands of years to reach the sea. Water is also contained in reservoirs such as lakes and the polar ice caps , but about 97% of the earth's water is contained in the oceans. The sun's energy heats and evaporates water from the ocean surface. On average, evaporation exceeds precipitation over the oceans, while precipitation exceeds evaporation over land masses. Horizontal air motions can transfer evaporated water to areas where clouds and precipitation subsequently form, completing the circle which can then begin again.

The distribution of precipitation is not uniform across the earth's surface, and varies with time of day, season and year. The lifting and cooling that produces precipitation can be caused by solar heating of the earth's surface, or by forced lifting of air over obstacles or when two different air masses converge. For these reasons, precipitation is generally heavy in the tropics and on the upwind side of tall mountain ranges. Precipitation over the oceans is heaviest at about 7°N latitude (the intertropical convergence zone), where the tradewinds converge and large thunderstorms frequently occur. While summer is the "wet season" for most of Asia and northern Europe , winter is the wettest time of year for Mediterranean regions and western North America . Precipitation is frequently associated with large-scale low-pressure systems (cyclones) at mid-latitudes.


Human influences on precipitation

Precipitation is obviously important to humankind as a source of drinking water and for agriculture. It cleanses the air and maintains the levels of lakes, rivers, and oceans, which are sources of food and recreation. Interestingly, human activity may influence precipitation in a number of ways, some of which are intentional, and some of which are quite unintentional. These are discussed below.


Cloud seeding

The irregular and frequently unpredictable nature of precipitation has led to a number of direct attempts to either stimulate or hinder the precipitation process for the benefit of humans. In warm clouds, large hygroscopic particles have been deliberately introduced into clouds in order to increase droplet size and the likelihood of collision and coalescence to form raindrops. In cold clouds, ice nuclei have been introduced in small quantities in order to stimulate precipitation by encouraging the growth of large ice crystals; conversely, large concentrations of ice nuclei have been used to try to reduce numbers of supercooled droplets and thereby inhibit precipitation formation. Silver iodide, which has a crystalline structure similar to that of ice, is frequently used as an ice nucleus in these "cloud seeding" experiments. Although certain of these experiments have shown promising results, the exact conditions and extent over which cloud seeding works and whether apparent successes are statistically significant is still a matter of debate.


Acid rain

Acid rain is a phenomenon that occurs when acidic pollutants are incorporated into precipitation. It has been observed extensively in the eastern United States and northern Europe. Sulfur dioxide , a gas emitted by power plants and other industries, can be converted to acidic sulfate compounds within cloud droplets. In the atmosphere, it can also be directly converted to acidic particles, which can subsequently act as CCN or be collected by falling raindrops. About 70 megatons of sulfur is emitted as a result of human activity each year across the planet . (This is comparable to the amount emitted naturally.) Also, nitrogen oxides are emitted by motor vehicles, converted to nitric acid vapor, and incorporated into clouds in the atmosphere.

Acidity is measured in terms of pH , the negative logarithm of the hydrogen ion concentration ; the lower the pH, the greater the acidity. Water exposed to atmospheric carbon dioxide is naturally slightly acidic, with a pH of about 5.6. The pH of rainwater in remote areas may be as low as about 5.0 due to the presence of natural sulfate compounds in the atmosphere. Additional sulfur and nitrogen containing acids introduced by anthropogenic (human-induced) activity can increase rainwater acidity to levels that are damaging to aquatic life. Recent reductions in emissions of sulfur dioxide in the United Kingdom have resulted in partial recovery of some affected lakes.


Greenhouse effect

Recent increases in anthropogenic emissions of trace gases (for example, carbon dioxide, methane, and chloroflourocarbons) have resulted in concern over the so-called greenhouse effect . These trace gases allow energy in the form of sunlight to reach the earth's surface, but "trap" or absorb the infrared energy (heat ) that is emitted by the earth. The heat absorbed by the atmosphere is partially re-radiated back to the earth's surface, resulting in warming. Trends in the concentrations of these greenhouse gases have been used in climate models (computer simulations) to predict that the global average surface temperature of the earth will warm by 3.6–10.8°F (2–6°C) within the next century. For comparison, the difference in average surface temperature between the Ice Age 18,000 years ago and present day is about 9°F (5°C).

Greenhouse warming due to anthropogenic activity is predicted to have other associated consequences, including rising sea level and changes in cloud cover and precipitation patterns around the world. For example, a reduction in summertime precipitation in the Great Plains states is predicted by many models and could adversely affect crop production. Other regions may actually receive higher amounts of precipitation than they do currently. The level of uncertainty in these model simulations is fairly high, however, due to approximations that are made. This is especially true of calculations related to aerosol particles and clouds. Also, the natural variability of the atmosphere makes verification of any current or future trends extremely difficult unless actual changes are quite large.


Effects of particulate pollution on cloud microphysics

As discussed above, gas-phase pollutants such as sulfur dioxide can be converted into water-soluble particles in the atmosphere. Many of these particles can then act as nuclei of cloud droplet formation. Increasing the number of CCN in the atmosphere is expected to change the characteristics of clouds. For example, ships' emissions have been observed to cause an increase in the number of droplets in the marine stratus clouds above them. If a constant amount of liquid water is present in the cloud, the average droplet size will be smaller. Higher concentrations of smaller droplets reflect more sunlight, so if pollution-derived particles alter clouds over a large enough region, climate can be affected. Precipitation rates may also decrease, since droplets in these clouds are not likely to grow large enough to precipitate.

See also Seasons; Thunderstorm; Weather modification.


Resources

books

Mason, B.J. Acid Rain: Its Causes and its Effects on InlandWaters. Oxford: Clarendon Press, 1992.

Rogers, R.R., and M.K. Yau. A Short Course in Cloud Physics. Oxford: Pergamon Press, 3rd Edition, 1989.

Wallace, John M., and Peter Hobbs. Atmospheric Science: An Introductory Survey. Orlando: Academic Press, Inc., 1977.


periodicals

Schneider, Stephen. "The Greenhouse Effect: Science and Policy." Science 243 (1989): 771-781.


Cynthia Twohy Ragni

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aerosol particles

—Solid or liquid particles suspended in the air.

Cold cloud

—A cloud that exists, at least in part, at temperatures below 32°F (0°C).

Hailstone

—Precipitation that forms when supercooled droplets collide with ice and freeze.

Mixed cloud

—A cloud that contains both liquid water and ice.

Supercooled

—Water than exists in a liquid state at temperatures below 32°F (0°C).

Virga

—Precipitation that evaporates before reaching the ground.

Warm cloud

—A cloud that exists entirely at temperatures warmer than 32°F (0°C).

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