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 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., 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 3.6–10.8°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 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

precipitation Precipitation—water falling to the ground—is one of the single most obvious elements of the weather. The presence of precipitation is quickly felt once it occurs, and its total duration in time at any location may consequently seem to be far greater than it actually is. However, even in the wettest parts of the tropics, or in exposed maritime areas in the temperate zones, there is at least as much ‘dry’ time as there is ‘wet’, and frequently much more. In truth, very few clouds precipitate.

Precipitation may take a variety of different forms (rain, snow, sleet, etc.); it may show considerable variation in magnitude (intensity), from the heaviest in thunderstorms or tropical cyclones to the lightest drizzle of windward coasts and hills; and it may be fleeting in showers, be continuous, or be intermittent. ‘Clouds’ at ground level (fog or mist) can also yield precipitation when the wind drives water droplets on to solid obstacles, such as buildings or trees. These general characteristics of precipitation are used in its classification. They also often give vital clues to the processes behind its generation. These processes are still imperfectly understood, and in all of them there is the major physical problem of generating particles, large enough to fall to the ground, from the very tenuous collection of small particles of ice or water which make up clouds. Surviving the fall to the surface is a further problem in the unsaturated air beneath the cloud.

Precipitation is conventionally measured using rain gauges which conform to a standard national pattern. These are read daily, generally at 09.00hours local time, to the nearest 0.1mm depth. In many countries such records extend well back into the nineteenth century. Some gauges are also capable of measuring precipitation automatically at much shorter intervals (e.g. every hour). The gauges themselves have to be carefully sited away from obstructions to obtain a ‘realistic’ measure of amount, but they are subject to error, and each represents only a minute ‘sample’ of an often huge surrounding area. Precipitation is highly variable at the microscale in both space and time. The reliance on gauge measurement has until recently restricted precipitation measurement to populated land areas, but during the 1980s and 1990s rapid advances were made in the field of radar and satellite ‘sensing’ of precipitation. Many nations now operate a network of radar installations, enabling good estimates to be made of precipitation occurrence and movement every 15minutes, and the newest satellite technology provides good estimates of precipitation amount over the two-thirds of the Earth's surface covered by water.

At lower levels, in the tropical latitudes, and during the warmer parts of the year in the temperate zone, most precipitation is in the liquid form—rain or drizzle—once it hits the ground. The distinction between the two is mainly one of size: drizzle drops have diameters of less than 0.5 mm and are close together. Raindrops are equal to, or greater than, 0.5mm. Intensities greater than 4.0 millimetres per hour (mm h⁻¹) indicate heavy rain, and intensities less than 0.5mm h⁻¹ slight rain; moderate rain fills the gap between these limits. When the rain falls as showers, a different range of intensities is used, because of the nature of ‘short, sharp showers’: ‘violent’, greater than 50mmh⁻¹; heavy, 10−50mmh⁻¹; moderate, 2−10mmh⁻¹; slight, less than 2mmh⁻¹. The definitions for drizzle intensity are more descriptive: ‘thick’ drizzle ‘definitely impairs visibility and accumulates at a rate of up to 1 mm h⁻¹’; moderate drizzle ‘causes windows and road surfaces to stream with moisture’; slight drizzle is ‘readily detected on face, but produces very little runoff’.

At higher elevations, in high latitudes, and also during the colder part of the year in temperate areas, solid precipitation is more common. Typically this falls as snow, but there is a wide variety of other terms for solid precipitation based on the size and structure of the individual particles. Snowflakes comprise the familiar loose aggregates of ice crystals which often adopt a hexagonal and branched form; there are also snow and ice pellets, snow grains, granular snow (graupel), and ice prisms. The distinction between snow and ice is that snow particles are always opaque. The intensity ranges for snow are ten times those for rain: 10 cm of freshly fallen snow is equivalent to 10 mm of rain. At times, formerly solid precipitation will have melted only partially by the time it reaches the ground surface, or snow and rain can fall simultaneously, that is, as sleet.

Another important category of solid precipitation is that of hail. Hailstones are generated by a unique set of processes that operate in cumulonimbus clouds, which contain rapidly rising thermals of warm air and strong down draughts of cold air, through a considerable depth of atmosphere, and are sometimes associated with thunder, lightning, and tornadoes. When looked at in section, hailstones show a series of alternately opaque and clear ice rings. Each opaque layer represents passage through a part of the cloud where freezing was very rapid, trapping air bubbles. Each clear layer represents times when air bubbles were able to escape when freezing was slower. Together, pairs of rings mark the completion of a cycle of vertical movement within the cloud, during which the hailstone is alternately carried rapidly to high levels in the cloud on top of rising thermals, and then downwards within equally strong down draughts. Freezing is very rapid in the upper layers of the cloud and much slower at lower levels.

Precipitation develops within clouds in which there is notable and sustained uplift. Under such conditions the clouds become thick enough for one or more of a variety of processes to operate and cause the precipitation. Mechanisms for uplift are present at the local scale within shower and thunderstorm clouds (cumulus and cumulonimbus), where air is forced upwards over hills and mountain ranges or along locally developed zones of convergence, such as sea-breeze fronts (‘forced uplift’ or ‘forced convection’), and at the larger scale, within weather systems such as temperate-latitude frontal depressions and tropical cyclones. These synoptic weather features can also incorporate local-scale uplift.

Precipitation thus dominantly occurs in parts of the world that are subject to the development and passage of major synoptic weather features within the temperate westerly belts in both hemispheres, in some parts of the tropics where cyclones can form, and in areas prone to sustained daytime surface heating, generating strong thermals, or the windward slopes of higher ground. Such processes may also typically show marked seasonal or diurnal variation, with the more intense thunderstorm or shower-type precipitation generally occurring in mid- to late afternoon, in the summer months in temperate zones, and within the humid tropics. The larger-scale development of temperate depressions favours the winter half of the year, whereas tropical cyclone activity peaks during the hottest months.

These weather systems are efficient generators of precipitation at varying spatial scales. More generally though, the range of conditions that must be satisfied to generate precipitation within a cloud is very exacting. The concentration of water droplets or ice particles within clouds is very low, and their size is extremely small. There is thus far more ‘space’ between adjacent particles than there is particle volume. This has important consequences for the potential generation of precipitation. Given their spacing, the opportunities for collision between particles are remote. Their size is such that many are more likely to be carried upwards by thermals, rather than to fall under the influence of gravity; and even if they were to fall from the cloud base they would rapidly evaporate once in the unsaturated air between cloud and ground. Some mechanisms are required that will significantly increase their size (and therefore their mass).

Early theories of precipitation assumed that as a cloud developed and grew in volume, the cloud droplets would simply continue to grow until some of them became large enough to fall as precipitation. For spherical cloud droplets the surface area increases very rapidly for a comparatively modest increase in radius. An increased surface area means that even if copious amounts of water vapour are available in the surrounding air, encouraging initial condensation and droplet growth, the supply is rapidly exhausted. Although the rate of growth for small droplets may be rapid, for large droplets to grow to raindrop size may in theory take days, which is more than the lifetime of the cloud.

There are two basic processes that enable cloud particles to mature into fully fledged precipitation. The first entails collision between cloud particles. This is afforded by the considerable differential horizontal and, particularly, vertical motion within clouds, and the varying rates of fall (or rise) of particles within the cloud according to their mass (i.e. size). Small particles in static air will fall only very slowly towards the ground under gravity. Initially they will accelerate, but they will soon reach a constant terminal velocity when the air resistance around them offsets their downward acceleration. For larger particles this terminal velocity will be reached later, and will be higher. The terminal velocity for a droplet of 0.05 mm radius will be 0.25 metres per second (m s⁻¹), and that for a 0.001 mm droplet will be 0.0001 m s⁻¹. For a 2.5 mm raindrop it is 9.1 m s⁻¹. Smaller particles will also be more easily carried aloft by updraughts; larger ones, less so. Larger particles fall more rapidly towards the surface, the smaller less so. The opportunity for collision between particles is thus considerably increased, with larger particles sweeping up smaller particles in the downward direction and colliding with upward-moving ones. Where collision occurs between liquid droplets, the process is referred to as coalescence. Collision between solid particles, is called aggregation; between liquid and solid it is known as accretion.

The second process that generates precipitation within a cloud is commonly known as the Bergeron–Findeisen process, after the two meteorologists who independently evolved the theory in the 1930s. The process centres on the ability of liquid and solid cloud particles to coexist at temperatures between 0 °C and −40 °C. All cloud particles above 0 °C will be liquid, and all below −40 °C will be solid. The number of liquid (supercooled) droplets decreases below 0°C. Between −10 and −30°C there is commonly a mixture of both solid and supercooled liquid particles. Many clouds (particularly those with considerable vertical extent such as conventional clouds) contain a deep layer between these two temperatures. In thunderstorm clouds, the upper layers are below −40°C, while the lower layers are frequently above freezing point. When significant quantities of supercooled droplets are available, nearby solid ice particles will grow at the expense of the liquid droplets. This occurs because air which is saturated with respect to ice is unsaturated with respect to water. Water will initially evaporate from a droplet and its size will be reduced. Because the air around the ice is now supersaturated, deposition occurs on to the surface of the ice particle, increasing its bulk. The air near the droplet is now once again unsaturated, and the process continues, allowing ice particles to grow to considerable size at the expense of water droplets.

Within typical clouds a combination of collision and Bergeron–Findeisen processes can occur. For ‘warm’ clouds (exclusively above 0 °C) only coalescence operates. Outside the tropics, warm clouds are shallow, so that only slight rain or drizzle will result. For ‘cold’ clouds, however, all processes can operate, and the considerable vertical motion within cumulonimbus clouds, with their tops at temperatures below −40 °C, and their lower layers perhaps above 0 °C, affords the opportunity for particles to circulate within them many times over, growing continually, sometimes by collision, sometimes accreting droplets, sometimes by aggregation, and often producing hailstones of a significant size. In addition, the upper parts of the cloud may continually be ‘seeding’ the higher-temperature parts of the cloud beneath with ice particles, which ‘feed’ on this raw material. At the same time, new liquid water may be swept up into the same layer, perpetuating the growth of particles by the Bergeron–Findeisen process.

Graham Sumner

Bibliography

Meteorological Office (1972) Observers' handbook. HMSO Publications, London.
Sumner, G. (1988) Precipitation: process and analysis. John Wiley and Sons, Chichester.

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.

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: AgNO ₃ +Cl ^- →NO ₃^- +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.

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.

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.

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.

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.

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.

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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