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The how and why of rain

By Ross Atkin / April 25, 2000



April showers bring May flowers, indeed. But where does rain come from, and where does it go?

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Actually, the same water falls as rain again and again. The water we use today was around in Roman times. Earlier, in fact: Dinosaurs may have lapped the water you just brushed your teeth with.

Liquid water evaporates into the atmosphere from oceans, rivers, and puddles. It evaporates from wet laundry drying in the sun. "Evaporate" means "to turn into a vapor, or gas" - water vapor.

The water vapor turns back into a liquid and falls to earth as rain (or snow or sleet or hail), and then evaporates again - over and over. Scientists call this the "hydrologic" cycle. "Hydro" means "water" in Greek. But before water can fall back to earth as rain, you need clouds. And before you can have clouds, you need ... dust.

Water molecules need something to latch onto in order to form what scientists call "cloud droplets." Cloud droplets are what clouds are made of:microscopic globules of water suspended in the air.

Dust particles serve as tiny space stations that attract water molecules. The dust particles become the "nuclei" (NUKE-lee-eye) or centers for cloud droplets.

Raindrops are just grown-up cloud droplets. Cloud droplets grow through condensation, and by bumping into one another and merging.

Condensation is what happens on the outside of a drinking glass when it's filled with icy water on a warm day. Humid air hits the cold glass, and water droplets form on dust particles.

Warm, moist air rises through the atmosphere, just as hot-air balloons rise. As the air rises, it cools: Cooler air can't hold as much water vapor, so the excess water vapor condenses. Cloud droplets form.

Mary Miller, a weather expert at the Exploratorium museum in San Francisco, describes what happens next: The wind acts as a giant mixer, smashing water droplets together. As cloud droplets hit other water particles, "they kind of glom onto them and grow. After a while, those drops get too heavy to stay in the cloud. Then they fall."

It's raining!

You may also be surprised to learn what raindrops look like. They are not teardrop-shaped, as they are often pictured. When they are still small enough to be suspended in clouds, raindrops are tiny spheres. As they get bigger and fall, they flatten on the bottom because they're hitting the air. They look like hamburgers - flat on the bottom and rounded on top. As they fall, raindrops oscillate (jiggle and wobble). They get flatter and rounder by turns. They may split into smaller drops.

How fast does rain go?

The smaller the raindrop, the weaker the pull of gravity on it, and the more slowly it falls. A drizzly rain may descend at only 70 cm per second (2.5 km per hour, or 1-1/2 miles per hour). Big, heavy raindrops fall at from 21 to 36 km per hour (13 to 22 miles per hour). Rain can't fall much faster than that, though the wind may blow it around pretty hard.

As the rain falls to earth, people without umbrellas run for cover. But is that the best idea? The scientists we asked weren't sure.