Rain

I

Introduction

Rain, precipitation of liquid drops of water from clouds. Raindrops generally have a diameter greater than 0.5 mm (0.02 in). They range in size up to about 3 mm (about 0.13 in) in diameter, and their rate of fall increases, up to 7.6 m (25 ft) per second with their size. Larger drops tend to be flattened and broken into smaller drops by rapid fall through the air. The precipitation of smaller drops, called drizzle, often severely restricts visibility but usually does not produce significant accumulations of water. Other forms of precipitation include water frozen as snow, hail, or sleet. (See also Weather.)

Amount or volume of rainfall is expressed as the depth of water that collects on a flat surface, and is measured in a rain gauge to the nearest 0.25 mm (0.01 in). Rainfall is classified as light if not more than 2.5 mm (0.10 in) per hour, heavy if more than 7.50 mm (more than 0.30 in) per hour, and moderate if between these limits.

Many natural environments and ecosystems such as rain forests and wetlands depend on natural rain cycles, as do grasslands and savannas. From a human perspective, rain is essential for agriculture in many regions of the world, especially where irrigation is not practical or available. Lack of rain leads to drought, which can damage or destroy crops and create conditions for destructive wildfires. Large amounts of rain can lead to flooding, landslides, and erosion. Changes in rain patterns may be caused by periodic weather phenomena such as El Niño, or by human activities leading to global warming, desertification, or deforestation.

II

Process of Precipitation

Air masses acquire moisture on passing over warm bodies of water, or over wet land surfaces. The moisture, or water vapor, is carried upward into the air mass by turbulence and convection (see Heat Transfer). The lifting required to cool and condense this water vapor results from several processes, and study of these processes provides a key for understanding the distribution of rainfall in various parts of the world. For water vapor to first form clouds that can then produce precipitation requires tiny airborne particles that water can condense around. Particulates that can serve as condensation nuclei include dust, soot, pollutants, sea salt, volcanic material, and even microorganisms.

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The phenomenon of lifting, associated with the convergence of the trade winds (see Wind), results in a band of copious rains near the equator. This band, called the intertropical convergence zone (ITCZ), moves northward or southward with the seasons. A wind pattern called a monsoon occurs in some tropical regions and is associated with high amounts of rainfall. In higher latitudes much of the lifting is associated with moving cyclones (see Cyclone), often taking the form of the ascent of warm moist air, over a mass of colder air, along an interface called a front. Lifting on a smaller scale is associated with convection in air that is heated by a warm underlying surface, giving rise to showers and thunderstorms. The heaviest rainfall over short periods of time usually comes from such storms. Air may also be lifted by being forced to rise over a land barrier, with the result that the exposed windward slopes have enhanced amounts of rain while the sheltered, or lee, slopes have little rain.

III

Average Rainfall

In the United States the heaviest average rainfall amounts, up to 1,778 mm (70 in) or more, are generally experienced in the Southeast, where air masses from the tropical Atlantic and Gulf of Mexico are lifted frequently by cyclones and by convection. Moderate annual accumulations, from about 762 to 1,270 mm (30 to 50 in), occur throughout the eastern United States, and are caused by cyclones in winter and convection in summer. The central plains, being farther from sources of moisture, have smaller annual accumulations, about 381 to 1,016 mm (15 to 40 in), mainly from summer convective storms. The southwestern United States is dominated by widespread descent of air in the subtropical Pacific anticyclone; rainfall is light, less than 254 mm (less than 10 in), except in the mountainous regions. The northwestern states are affected by cyclones from the Pacific Ocean, particularly during the winter; but rainfall is moderate.

Some of the world’s heaviest average rainfall, about 10,922 mm (about 430 in) per year, occurs at Cherrapunji, in northeastern India, where moisture-laden air from the Bay of Bengal is forced to rise over the Khāsi Hills of Assam State. As much as 26,466 mm (1,042 in), or 26 m (87 ft), of rain have fallen there in one year. Other extreme rainfall records have included nearly 1,168 mm (nearly 46 in) of rain in one day during a typhoon at Baguio, Philippines; 304.8 mm (12 in) within one hour during a thunderstorm at Holt, Missouri; and 62.7 mm (2.48 in) over a 5-minute period at Portobelo, Panama.

IV

Artificial Precipitation

Despite the presence of moisture and lifting, clouds sometimes fail to precipitate rain. The study of precipitation processes to learn how single raindrops are produced out of a million or so minute droplets inside clouds has identified at least two precipitation processes. In one, water drops at subfreezing temperatures form into ice crystals that later fall into warmer layers and melt. In another process, smaller water droplets collect into larger drops that fall at a higher speed.

Efforts to reproduce such processes artificially have led to weather modification operations such as cloud seeding (see Meteorology). Cloud seeding commonly uses silver iodide crystals as condensation nuclei to form water droplets that can fall as rain or as snow. These efforts may have limited success, since most areas with deficient rainfall are dominated by air masses that have either inadequate moisture content or inadequate elevation, or both.

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