Precipitation

Precipitation consists of particles of liquid or frozen water that fall from clouds toward the ground surface. Thus, precipitation links the atmosphere with the other reservoirs of the global hydrologic cycle, replenishing oceanic and terrestrial reservoirs. In addition, precipitation is the ultimate source of freshwater for irrigation, industrial consumption, and supplies of drinking water.

Cloud Particles

Precipitation consists of liquid or frozen particles of water that fall from clouds and normally reach the ground surface. Under certain conditions, however, a type of precipitation known as virga forms and falls normally, but evaporates before reaching the ground. The most familiar types of precipitation are rain and snow. Perhaps surprisingly, most clouds, even those associated with large storm systems, do not produce precipitation. A typical cloud particle is about one-millionth the size of a raindrop. Special circumstances are required for the extremely small water droplets or ice crystals that compose a cloud to grow into raindrops or snowflakes.

Cloud particle diameters are typically in the range of 2 to 50 micrometers, with one micrometer being one-millionth of a meter. They are so small that they remain suspended within the atmosphere unless they vaporize or somehow undergo considerable growth. Upward-directed air currents, or updrafts, are usually strong enough to prevent cloud particles from leaving the base of a cloud. Even if cloud droplets or ice crystals descend from a cloud, their fall rates are so slow that they quickly vaporize in the relatively dry air under the cloud. In order to precipitate, therefore, cloud particles must grow sufficiently massive that they counter updrafts and survive thousands of meters of descent to the ground surface. Cloud physicists have identified two processes whereby cloud particles grow large enough to precipitate: the Bergeron process and the collision-coalescence process.

The Bergeron and Collision-Coalescence Processes

Most precipitation originates via the Bergeron process, named for the Scandinavian meteorologist Tor Bergeron, who, in about 1930, first described the process. It occurs within cold clouds at a temperature below freezing (0 degrees Celsius). Cold clouds are composed of ice crystals or supercooled water droplets or a mixture of the two. Supercooled water droplets are tiny drops that remain liquid at temperatures below their normal freezing point. Bergeron discovered that precipitation is most likely to fall from cold clouds composed of a mixture in which supercooled water droplets at least initially greatly outnumber ice crystals. In such a circumstance, ice crystals grow rapidly while supercooled water droplets vaporize. As ice crystals grow, their fall rates within the cloud increase. They collide and merge with smaller ice crystals and supercooled water droplets in their paths and thereby grow still larger. Eventually, the ice crystals become so heavy that they fall out of the cloud base. If the air temperature is below freezing during most of the descent, the crystals reach the surface as snowflakes. If, however, the air below the cloud is above freezing, the snowflakes melt and fall as raindrops.

Growth of ice crystals at the expense of supercooled water droplets in the Bergeron process is linked to the difference in the rate of escape of water molecules from an ice crystal versus a water droplet. Water molecules are considerably more active in the liquid phase than in the solid phase. Hence, water molecules escape from water droplets more readily than they do from ice crystals. Within a cold cloud, air that is saturated for water droplets is actually supersaturated for ice crystals. Consequently, water molecules diffuse from the water droplets and deposit on the ice crystals. That is, the water droplets vaporize and release water molecules to the air as the ice crystals grow by accruing those water molecules from the air.

The collision-coalescence process occurs in warm clouds (clouds at temperatures above 0 degrees Celsius). Such clouds are composed entirely of liquid water droplets. Precipitation may develop if the range of cloud droplet sizes is broad. Larger cloud droplets have greater fall velocities than do smaller droplets, as they are less affected by upwelling air currents. As a result, larger droplets collide and coalesce with smaller droplets in their paths. Collision and coalescence are repeated a multitude of times until the droplets become so large and heavy that they fall from the base of the cloud as raindrops. Since the force of upwelling air currents varies, the forming droplets may be pushed back up to higher altitudes time and time again, becoming larger each time as their fall path through the cloud is extended.

Once a raindrop or snowflake leaves a cloud, it enters drier air, a hostile environment in which some of the precipitation vaporizes. In general, the longer the journey to the ground surface and the drier the air beneath the cloud, the greater the amount of rain or snow that returns to the atmosphere as vapor. It is understandable, then, why highlands receive more precipitation than do lowlands, which are hundreds to thousands of meters farther from the base of the clouds.

Types of Precipitation

Precipitation occurs in a variety of liquid and frozen forms. Besides the familiar rain and snow, precipitation also occurs as drizzle, freezing rain, ice pellets, and hail. Drizzle consists of small water drops less than 0.5 millimeter in diameter that drift very slowly downward to the ground. The relatively small size of drizzle drops stems from their origin in low stratus clouds or fog. Such clouds are so shallow that droplets originating within them have a limited opportunity to grow by coalescence.

Rain falls most often from thick nimbostratus and cumulonimbus (thunderstorm) clouds. The bulk of rain originates as snowflakes or hailstones, which melt on the way down as they enter air that is warmer than 0 degrees Celsius. Because rain originates in thicker clouds, raindrops travel farther than does drizzle, and they undergo more growth by coalescence. Most commonly, raindrop diameters range from 0.5 to 5 millimeters; beyond this range, drops are unstable and break apart into smaller drops. Freezing rain (or freezing drizzle) develops when rain falls from a relatively mild air layer onto the ground-level objects that are at temperatures below freezing. The drops become supercooled, then freeze immediately on contact with subfreezing surfaces. Freezing rain forms a layer of ice that sometimes grows thick and heavy enough to bring down tree limbs, power lines, and grid towers; disrupt traffic; and make walking or transportation hazardous.

Snow is an assemblage of ice crystals in the form of flakes. Although it is said that no two snowflakes are identical, all snowflakes have hexagonal (six-sided) symmetry. Snowflake form varies with air temperature and water vapor concentration and may consist of flat plates, stars, columns, or needles. Snowflake size also depends in part on the availability of water vapor during the crystal growth process. At very low temperatures, the water vapor concentration is low so that snowflakes are relatively small. Snowflake size also depends on collision efficiency as the flakes drift toward the ground. At temperatures near freezing, snowflakes are wet and readily adhere to each other after colliding, so flake diameters may eventually exceed 5 centimeters. Snow grains and snow pellets are closely related to snowflakes. Snow grains originate in much the same way as drizzle, except that they are frozen. Their diameters are generally less than 1 millimeter. Snow pellets are soft conical or spherical white particles of ice with diameters of 1 to 5 millimeters. They are formed when supercooled cloud droplets collide and freeze together, and they may accompany a fall of snow.

Ice pellets, often called sleet, are frozen raindrops. They develop in much the same way as does freezing rain, except that the surface layer of subfreezing air is so deep that raindrops freeze before striking the ground. Sleet can be distinguished readily from freezing rain because sleet bounces when striking a hard surface, whereas freezing rain does not.

Hail consists of rounded or irregular pellets of ice, often characterized by an internal structure of concentric layers resembling the interior of an onion. Hail develops within severe thunderstorms as vigorous updrafts propel ice pellets upward into the higher reaches of the cloud. It is not unusual for clouds in severe thunderstorms to reach altitudes of more than 10 kilometers. Along the way, ice pellets grow via coalescence with supercooled water droplets and eventually become too heavy to be supported by updrafts. The ice pellets then descend through the cloud, exit the cloud base, and enter air that is typically above freezing. As ice pellets begin to melt, those that are large enough may survive the journey to the ground as hailstones. Most hail consists of harmless granules of ice less than 1 centimeter in diameter, but violent thunderstorms may spawn destructive hailstones the size of golf balls or larger. Hail is usually a spring and summer phenomenon that can be particularly devastating to crops as it shreds leaves and flowers, breaks fruit loose from the branches, and can even render damage to agricultural equipment.

Changes in Precipitation Chemistry

Over the past few decades, considerable concern has been directed at the environmental impact of changes in the chemistry of precipitation. Water vapor in the atmosphere is molecularly pure water. The global hydrologic cycle purifies water through what is essentially the process of distillation, and the droplets of liquid water that form from the vapor are also composed of molecularly pure water, which has a pH value of 7. But as raindrops and snowflakes fall from clouds to the ground, they dissolve and interact with pollutants in the air. In this way, the chemistry of precipitation is altered. Rain is normally slightly acidic because it dissolves atmospheric carbon dioxide, producing a very weak solution of carbonic acid having a pH only slightly less than 7. Where air is polluted with oxides of sulfur and oxides of nitrogen, however, these gases interact with moisture in the atmosphere to produce droplets of sulfuric acid and nitric acid solutions. These acidic droplets greatly increase the acidity of precipitation. Precipitation that falls through such polluted air may become orders of magnitude more acidic than normal, and was once measured in Scotland, in 1974, as having a pH of only 2.4, more than ten thousand times more acidic than normal, unpolluted rainfall.

Field studies have confirmed a trend toward increasingly acidic precipitation in the form of rain and snow over the eastern one-third of the United States. Much of this upswing in acidity can be attributed to acid rain precursors emitted during fuel combustion. Coal-burning for electric power generation is the principal source of sulfur oxides, while high-temperature industrial processes and motor vehicle engines produce nitrogen oxides. Where acid rains fall on soils or bedrock that cannot neutralize the acidity, lakes and streams become more acidic. Excessively acidic lake or stream water disrupts the reproductive cycles of fish and has numerous other negative environmental effects. Acid rains leach metals (such as aluminum) from the soil, washing them into lakes and streams, where they may harm fish and aquatic plants.

Study of Precipitation

Precipitation is collected and measured with essentially the same device that has been used since the fifteenth century: a container open to the sky. The standard US National Weather Service rain gauge consists of a cone-shaped funnel that directs rainwater into a long, narrow cylinder that sits inside a larger cylinder. The narrow cylinder magnifies the scale of accumulating rainwater so that rainfall can be resolved into increments of 0.01 inch. (Rainfall of less than 0.005 inch is recorded as a “trace.”) Rainwater that accumulates in the inner cylinder is measured against a graduated scale. Rainfall is measured at some fixed time once every twenty-four hours, and the gauge is then emptied.

With regard to snow, scientists are interested in measuring snowfall during each twenty-four-hour period between observations, as well as the meltwater equivalent of that snowfall, and the depth of snow on the ground at each observation time. New snowfall is usually collected on a simple board that is placed on top of the old snow cover. When new snow falls, the depth is measured to the board; the board is then swept clean and moved to a new location. The meltwater equivalent of new snowfall can be determined by melting the snow collected in a rain gauge (from which the funnel has been removed). Snow depth is usually measured with a special yardstick or meterstick. In mountainous terrain where snowfall is substantial, it may be necessary to use a coring device to determine snow depth (and meltwater equivalent). Snow depth is determined at several representative locations and then averaged.

The average density of fresh-fallen snow is 0.1 gram per cubic centimeter. As a general rule, 10 centimeters of fresh snow melts down to 1 centimeter of rainwater. This ratio varies considerably depending on the temperature at which the snow falls. “Wet” snow falling at surface air temperatures at or above 0 degrees Celsius has a much greater water content than does “dry” snow falling at surface air temperatures well below freezing. The ratio of snowfall to meltwater may vary from 3:1 for very wet snow to 30:1 for dry, fluffy snow.

Monitoring the timing and rate of rainfall is often desirable, especially in areas prone to flooding. Hence, some rain gauges provide a cumulative record of rainfall. In a weighing-bucket rain gauge, the weight of accumulating rainwater (determined by a spring balance) is calibrated as water depth. Cumulative rainfall is recorded continuously by a device that either marks a chart on a clock-driven drum or sends an electrical pulse to a computer or magnetic tape. During subfreezing weather, antifreeze in the collection bucket melts snow as it falls into the gauge so that a cumulative meltwater record is produced.

Both rainfall and snowfall are notoriously variable from one place to another, especially when produced by showers or thunderstorms. The emplacement of a precipitation gauge is particularly important in order to ensure accurate and representative readings. A level site must be selected that is sheltered from strong winds and is well away from buildings and vegetation that might shield the instrument. In general, obstacles should be no closer than about four times their height.

Significance

Without precipitation, Earth would have no freshwater and thus no life. When water vaporizes from oceans, lakes, and other reservoirs on the ground surface, all dissolved and suspended substances are left behind. Hence, water is purified (distilled) as it cycles into the atmosphere and eventually returns to the surface as freshwater precipitation. In this way, the global hydrologic cycle supplies the planet with an essentially fixed quantity of freshwater.

As the human population continues its rapid growth, however, demands on the globe’s fixed supply of freshwater are also increasing. At the same time, climate change has led to more extreme droughts in the twenty-first century, leading to an increased need for innovation to handle such challenges. In some areas, such as the semiarid American Southwest, water demand for agriculture and municipalities has spurred attempts to enhance precipitation locally through cloud seeding. Usually, cold clouds that contain too few ice crystals are seeded by aircraft with either silver iodide crystals (a substance with molecular properties similar to ice) or dry-ice pellets (solid carbon dioxide at a temperature of -78 degrees Celsius) in an effort to stimulate the Bergeron precipitation process.

Cloud seeding, although founded on an understanding of how precipitation forms, is not always successful and at best may enhance precipitation by perhaps 20 percent. The question remains whether the rain or snow that follows cloud seeding would have fallen anyway. Even if successful, cloud seeding may merely bring about a geographical redistribution of precipitation so that an increase in precipitation in one area is accompanied by a compensating reduction in a neighboring area. Cloud seeding that benefits agriculture in eastern Colorado, for example, might also deprive farmers of rain in the downwind states of Kansas and Nebraska. Still, scientists continued to experiment with cloud-seeding capabilities into the 2020s, introducing new technologies in an effort to make it more effective. For instance, researchers in the United Arab Emirates (UAE), which has emerged as a global leader in cloud-seeding efforts, developed nanoengineered materials capable of enhancing water vapor condensation, and thus potentially increasing rainfall in arid regions like the UAE. In the US, new cloud-seeding projects in Los Angeles attempted to combat the prolonged drought that struck the region in the 2010s.

The uncertainties of cloud seeding underscore the need for conservation of the planet’s freshwater resources. Conservation should entail not only strategies directed at wise use of freshwater but also measures to manage water quality. Abatement of water pollution not only reduces hazards to human health and aquatic systems but also increases the available supply of freshwater.

Principal Terms

Acid precipitation: rain or snow that is more acidic than normal, usually because of the presence of sulfuric and nitric acid

Bergeron process: precipitation formation in cold clouds, whereby ice crystals grow at the expense of supercooled water droplets

Cold cloud: a visible suspension of tiny ice crystals, supercooled water droplets, or both at temperatures below the normal freezing point of water

Collision-coalescence process: precipitation formation in warm clouds, whereby larger droplets grow through the merging of smaller droplets

Rain gauge: an instrument for measuring rainfall, usually consisting of a cylindrical container open to the sky

Supercooled water droplets: droplets of liquid water at temperatures below the normal freezing point of water

Warm cloud: a visible suspension of tiny water droplets at temperatures above freezing

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