What causes rain?

Dateline article| David Murphy|

The process of making rain begins when a mass of warm air rises above the earth's surface and cools. As air cools, the invisible gas-state water inside it (water vapor) begins to condense and turns into liquid water. Initially, these bits of water are so small and lightweight, they are suspended in the air and become clouds. At this point, they are called water droplets. But as the air becomes more and more moist, the water droplets beginning sticking to each other, forming larger and larger drops. Eventually, the drops get so big and heavy, they begin to fall. The smaller water drops evaporate before they hit the ground (they simply can't survive with all that dryer air rushing past them as they hurdle to earth.) Only the larger drops actually make it all the way down as rain and even those drops are often smaller than they were when they began to fall.

Sometimes, water vapor does not condense into liquid water until its reached very cold air. In this case, the water may turn into ice crystals and form snowflakes instead of water droplets. But eventually, the same thing happens. The ice crystals combine and become so heavy, they fall to earth. On the way down, they encounter warmer air, they melt and fall to the ground as rain.

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