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How rain is formed
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THE oceans are the chief source of rain, but lakes and rivers also contribute to it. The sun's heat evaporates (蒸发) the water. It remains in the atmosphere as an invisible vapour (水蒸气) until it condenses (冷凝), first into clouds and then into raindrops. Condensation happens when the air is cooled.
Air cools either through expansion (膨胀) or by coming into contact with a cool object such as a cold landmass (地块). When air passes over a cold object, it loses heat and its moisture condenses as fog, dew (露水), or frost (霜). Air also cools as it rises and expands. The water vapour in the cooling air condenses to form clouds and, sometimes, rain.
For raindrops to form there must be particulate (颗粒状的) matter in the air, such as dust or salt, at temperatures above freezing. These particles (分子) are called condensation nuclei (冷凝核). When the nuclei are cooled to temperatures below the freezing point, water condenses around them in layers. The particles become so heavy that they resist updrafts and fall through the clouds. If the updrafts are particularly strong, as in a thunderstorm, the rain nuclei may become very large and fall from the cloud as hail (冰雹). When the air temperature is at or below freezing all the way to the ground, the precipitation falls as snow.
Raindrops have diameters greater than 0.5 mm. When the drops are smaller, the precipitation is usually called drizzle (毛毛雨). Raindrops seldom have diameters larger than four mm, because as they increase in size they break up.
Mount Waialeale, Hawaii, with a 20-year annual average of 11,700 mm from tropical easterlies (东风带), is the wettest known point on the earth. The nearest competitor is Cherrapunji, India, with an annual average of 11,430 mm from the moist tropical monsoon (季风). Less than 250 mm and more than 1,500 mm per year represent approximate extremes of rainfall for all of the continents.
Rainfall is slight in the central regions of the subtropical anticyclones (副热带高气压带), which are therefore the desert regions of the earth. In parts of the desert no appreciable (明显的) rain has ever been observed.(SD-Agenices)
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