Rainfall is the amount of precipitation, in the form of rain (water from clouds), that descends onto the surface of Earth, whether it is on land or water. It develops when air masses travel over warm water bodies or over wet land surfaces. Atmospheric turbulence and convection carries the moisture, or water vapor, upward into air masses where they form clouds. The clouds eventually release this water vapor, which is dropped as rainfall.
Falling rain is usually composed of droplets with diameters of about 0.02 in (0.5 mm) or greater. A group of smaller raindrops is called drizzle. It is not considered rainfall when precipitation from clouds evaporates on the way down and fails to reach Earth's surface. The meteorological processes that control rainfall are studied intensely in order to learn more about how rainfall is distributed locally and globally.
Historical Background and Scientific Foundations
Rainfall is important in the hydrologic cycle, which is the continuing movement of water below, on, and above Earth's surface. This cycle involves water stored primarily in oceans, but also underground and in lakes, rivers, ice caps, and glaciers. Moisture evaporates from these containers and condenses in clouds, where it travels various distances before falling (precipitating) back to Earth as rainfall (and other forms of precipitation).
Water hitting Earth eventually finds its way to storage containers where it begins the cycle again. It is important to know that almost all water on Earth has neither been created nor lost over billions of years. The water present one billion years ago, for example, is being used today for drinking, bathing, cooking, and other uses.
Impacts and Issues
Clouds cannot be produced without dust particles of organic and inorganic substances such as bacteria, pollen, and silica.
These natural substances are suspended in the atmosphere and serve as carriers for the condensation of water vapor, what is called cloud condensation nuclei (CCN).
However, the chance of rainfall can be increased by artificially produced particles, such as pollution fromautomobile exhaust and industrial factories. When such particles like soot, smoke, and smog are produced, they lead to increased production of clouds, which directly increases the chances of rainfall.
Under natural conditions, several thousand of these CCN particles may be within an average cubic inch of atmosphere. However, in heavily industrialized areas, the addition of these artificial particles increases the total concentration of CCNs to over 50 million particles per cubic inch (over 800 million particles per cubic centimeter).
Artificially produced CCN particles can travel large distances and stay in the atmosphere for long periods of time. According to studies performed by the U.S. National Aeronautics and Space Administration (NASA), larger and more severe storms are produced when atmospheres over cities are polluted from human-made activities. These pollution-enhanced atmospheres produce more rainfall not only for the cities but also for areas downwind. In fact, scientists with NASA's Goddard Space Flight Center used space-based satellites to show changes in rainfall patterns and precipitation trends over Houston, Texas, between the 1940s and 1990s due to urban growth and industrial development.
WORDS TO KNOW
CLOUD CONDENSATION NUCLEI: Tiny solid airborne particles around which the water droplets or ice crystals that form clouds condense. By providing additional cloud condensation nuclei, aerosol pollution may influence cloud formation and thus climate.
CONVECTION: The rising of warm air from an object, such as the surface of Earth.
GLACIER: A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.04 mi2 (0.1 km2) in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of freshwater on Earth and is second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.
HYDROLOGIC CYCLE: The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30% of the mid-latitudes' heat transport from the equatorial to polar regions.
PRECIPITATION: Moisture that falls from clouds. Although clouds appear to float in the sky, they are always falling, the water droplets slowly being pulled down by gravity. Because their water droplets are so small and light, it can take 21 days to fall 1,000 ft (305 m) and wind currents can easily interrupt their descent. Liquid water falls as rain or drizzle. All raindrops form around particles of salt or dust. (Some of this dust comes from tiny meteorites and even the tails of comets.) Water or ice droplets stick to these particles, then the drops attract more water and continue getting bigger until they are large enough to fall out of the cloud. Drizzle drops are smaller than raindrops. In many clouds, raindrops actually begin as tiny ice crystals that form when part or all of a cloud is below freezing. As the ice crystals fall inside the cloud, they may collide with water droplets that freeze onto them. The ice crystals continue to grow larger, until large enough to fall from the cloud. They pass through warm air, melt, and fall as raindrops.
WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.
IN CONTEXT: NATURAL CHANGES IN ARID LANDS
Desertification is sometimes caused by natural influences other than lack of rainfall. This process has been ongoing for eons in some regions, in conjunction with long-term changes in climatic conditions, especially decreased precipitation. Until the twentieth century, humans were able to simply move their agricultural activity away from land rendered unusable by desertification. However, this strategy has been rendered less tenable by the immense population increase of humans during the past century, a change that has increased the attention paid to the degradation of once-productive drylands.
In desert areas, change usually occurs by some form of physical weathering. The wide diurnal temperature can make the modest amounts of moisture present in arid areas powerful weather factors through continual freezing and thawing cycles, which can result in micro-fracturing of rock. Winds often allow high levels of physical or frictional abrasion. Oxidation and other forms of chemical weathering produce familiar reddish desert “vamish.”
NASA and other scientific organizations continue studies on how natural and human-based particles affect rain and, thus, climate and weather.
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Jacobsen, Steven D., and Suzan van der Lee, eds. Earth's Deep Water Cycle. Washington, DC: American Geophysical Union, 2006.
Wang, Chunzai, Shang-Ping Xie, and James A. Carton, eds. Earth's Climate: The Ocean-Atmosphere Interaction. Washington, DC: American Geophysical Union, 2004.
“Are Cities Changing Local and Global Climates?” National Aeronautics and Space Administration, January 28, 2004. <http://www.nasa.gov/vision/earth/environment/Changing_Cities.html> (accessed October 31, 2007).