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 precipitate’s 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, K_sp, which in the case of silver chloride, is given by AgCl_(solid) ↔ Ag⁺_(aq) + Cl^-_(aq) K_sp= [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 solute’s 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 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 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 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 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 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 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 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 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 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 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 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 Earth’s 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 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 Earth will warm by 3.6–10.8^°F(2–6^°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.