The how and why of rain

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

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.

"If you're running, you're getting through the rain quicker," the Exploratorium's Ms. Miller says, "but you're hitting more drops." If you walk, you're out in the rain longer. "I don't think that question has been completely answered," she concludes. (Maybe you can think of experiments to do to find the answer.)

Nick Walker, the Weather Channel's "weather dude," says that running or walking isn't the issue. It depends on how hard it's raining. "I still run, though," he admits. "It's a reflex action. I want to get inside quickly."

A year ago, Mr. Walker lived in Seattle, a city known for its overcast skies and gentle rains. Now he lives in Atlanta, where it doesn't rain as often, but it does rain harder, and more, overall. Atlanta's summer downpours can easily deposit an inch or more of rain per hour.

Why this difference? In Seattle, moist air is constantly blowing in from the Pacific. As this air strikes the Cascade Range, just east of the city, it rises and cools. The moisture condenses on cloud droplets. The droplets grow and fall.

How mountains cast 'rain shadows'

The air continues over the Cascades. But now it's drier. Much less rain falls on the other side of the mountains. The Cascades have created a "rain shadow," a dry mirror image of drizzly Seattle.

In Atlanta, a broiling summer sun heats up humid air near the ground, causing it to rise straight up. This rising column of hot air forms billowing cumulus clouds that may suddenly unleash heavy, local showers.

Often, of course, rain occurs over a whole region. These storms are the result of a clash between large air masses. Where these masses meet is called a "front." When warm and cold air masses collide, you can get a forced lifting of air over a large region, says Peggy LeMone. She's a scientist with the National Center for Atmospheric Research in Boulder, Colo. The rising air can result in rain clouds blanketing the sky.

Ms. LeMone studies rain using different kinds of radar. She's also flown through a hurricane in a special plane for a firsthand look at what happens inside these huge storms.

Rainy questions remain

What's left to learn about rain? Researchers in Hawaii are trying to figure out how very shallow clouds there produce such big drops and heavy rains. Meteorologists also are fascinated by Cherrapunji, India, one of the rainiest places on earth because of the seasonal monsoons (storms that blow in from the Indian Ocean). And you may be wondering by now why Los Angeles, a coastal city with more than its share of airborne particles, gets so little rain. The answer is: wind. The prevailing direction of winds in the LA area is down, a common feature of desert areas. As the air sinks, it gets warmer and drier.

In the Chilean village of Caleta Chungungo, they've developed a way to address the lack of rain. A series of moisture-trapping "fog panels" made of fine mesh are set on the mountainside. The mesh screens trap fog droplets, providing much-valued water. To villagers, the same rain we shed with umbrellas and slickers is valued as liquid gold.

*Barry Keim, New Hampshire's state climatologist, also contributed to this story.

(c) Copyright 2000. The Christian Science Publishing Society