Rain - why does it rain? how do the clouds know when to rain?
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Why does it Rain?

Rain
Thanks to Rocbike for the great picture!

When the Earth first formed, about 4.5 billion years ago, it didn't rain here. Until about four billion years ago, Earth was too hot for raindrops to form, and water just hung in the air as steam without falling as rain. Then as the Earth's surface began to cool, in the Archaean era, huge clouds formed and began to rain huge rainstorms, making oceans wherever there were low places in the Earth's surface.

Nobody knows exactly what makes clouds begin to form raindrops and release their water as rain. Probably the cloud's water freezes on to tiny particles of dust or live bacteria inside the cloud, called cloud seeds, and these clumps gradually get big enough so that they are heavier, and then gravity pulls the frozen clump of ice toward the ground.

People sometimes try to make it rain if there is a drought by sending planes to shoot dust into clouds. We call this cloud seeding. Sometimes cloud seeding works and sometimes it doesn't - scientists are still studying how this works.

As the ice clump falls closer to the ground, usually the temperature is warmer, so the ice melts and falls as rain. But if it is cold, the ice may stay frozen, or re-freeze on its way down, and fall as freezing rain, or hail, or as snow.

After rain falls to earth, most of it gradually runs downhill into streams and then into rivers and from there to the oceans. Then the heat of the sun evaporates the water up into the sky again, where it forms clouds.

Different places on Earth have different rain patterns. In some places on Earth, like the Sahara Desert or parts of Peru, it hardly ever rains at all. In some places, like Seattle or London, it rains a little bit on many days of the year. In other places, like Mumbai, it hardly ever rains except for one or two months a year, when it rains very hard nearly every day - this is a monsoon rain.

Learn by Doing - Rain
More about Thunderstorms
More about Weather
Learn by doing - Seasons

Bibliography and further reading about weather:

Learn by Doing - Clouds
More about clouds
More about Thunderstorms
More about Weather
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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