How is Rain Formed

About 97 percent of the earth’s water is held in the oceans. The rest is, for the most part, locked up in glaciers or stored in lakes and aquifers. Of course, ocean water is not drinkable.Before ocean water becomes drinkable, it takes a long, complicated journey. First, it evaporates, becoming a gas—water vapor. Every year, heat from the sun draws up about 95,000 cubic miles [400,000 km3] of water from land and sea into the atmosphere. The atmosphere itself is “a system of almost unbelievable complexity” that extends more than 250 miles [400 km] into space. Our water is recycled in the 6 to 12 miles [10-20 km]closest to earth. This area, called the troposphere, is what the book Our Fragile Water Planet describes as “the region in contact with the earth’s surface, the realm of clouds, rain, snow, hurricanes, and tornadoes.”

The warmer the air, the more water it can hold. That is why your wash dries more quickly on a warm, windy day. The atmosphere in tropical regions holds the most water. ‘So how,’ you might ask, ‘is all this water moved to other places where it is needed?’ By the mighty wind systems that encircle the globe. They are created because of the way the earth spins on its axis and because some parts of the earth’s surface heat up more than others, keeping the atmosphere in a constant state of turbulence.

Our turbulent atmosphere contains huge air masses—great islands of air of more or less the same temperature. How big are they? They can cover an area of up to several million square miles. Warmer masses originate in the Tropics, and colder ones in arctic, or polar, regions. These air masses serve as massive atmospheric water transporters.

Another masterstroke of design is seen in the movement of water vapor in the atmosphere. It transfers heat from areas of oversupply, such as the Tropics, to areas of need. Otherwise, some parts of the earth would keep getting inexorably hotter and hotter.

Condensation

While water vapor performs vital functions in the atmosphere, it would obviously be of little use to us in watering the ground if it simply stayed up there. The atmosphere above the Sahara Desert, for example, contains considerable moisture, yet the region remains arid. How does atmospheric moisture get back to the earth? First, it condenses, reverting to liquid form.

You have likely seen water vapor condense in a bathroom when the warm air from a hot shower hits a colder window or mirror. Something similar happens when a parcel of air decreases in temperature as it rises into colder altitudes. What makes air rise? This can happen when a warm air mass is pushed higher by a denser, colder one. Sometimes air is forced upward by mountains. At other times, especially in tropical regions, it may be carried higher on convection currents.

‘But,’ you might ask, ‘what is there in the atmosphere for this vapor to condense on?’ The atmosphere is full of extremely small particles—such as smoke, dust, and sea salt. As a parcel of air cools, water vapor condenses on these tiny nuclei. Minute water droplets then become visible in the form of clouds.

However, this water does not immediately fall to earth. Why not? After all, water is 800 times denser than air. The answer is that each individual cloud droplet is so small and light that it can float in the air currents. Isn’t it amazing to realize that the small, fluffy cloud floating in the air above you may well contain from 100 to 1,000 tons of moisture?

Precipitation

Many clouds never do produce rain or, to be exact, precipitation. It is relatively easy to explain how water gets into the atmosphere and how clouds float in the sky. “The real difficulty,” says one writer, “is to explain how the water ever gets down” again.The Challenge of the Atmosphere.

It can take “a million or more cloud drops” to make one small raindrop. No one seems to have a totally satisfying answer as to what transforms these minute floating cloud droplets into the one billion tons or so of water that falls to the earth every minute of every day. Do the tiny cloud droplets simply merge to form larger raindrops? Sometimes they do. This likely accounts for raindrop formation in places such as the Tropics. But it does not begin to explain “the puzzle of raindrop formation” in places such as the Atlantic Coast of Ireland.

Here the tiny cloud droplets do not simply coalesce. By mechanisms not completely understood, they form tiny ice crystals. These group together to become “one of nature’s finest masterpieces”—the snowflake. As snowflakes grow larger and heavier, they overcome rising air currents and begin to fall to earth. If it is cold enough, they fall as snow—billions of them in an average snow shower. But if they fall through a layer of warm air, the snowflakes melt and become raindrops. Snow is thus not frozen rain. Rather, most rain, in temperate regions at least, begins as snow, which then melts as it falls to earth.

So after a journey that might well have been thousands of miles long, involving complicated processes not yet fully understood, the rain returns. Granted, it may interfere from time to time with your personal plans and pursuits. But this remarkable arrangement results in our having a never-ending water supply.

How Hailstones Are Formed

“Hail,” says the book Weather, “is the peculiar product of large, turbulent thunderclouds.” When cloud droplets condense on tiny nuclei in thunderclouds, they are sometimes caught in strong updrafts, which sweep them into higher freezing parts of the cloud. In these freezing temperatures, other droplets condense on the embryonic raindrop and instantly freeze. This process is repeated again and again, with the frozen raindrop traveling up and down into and out of the frozen layer. Each time, the frozen raindrop is gathering a new layer of ice, getting heavier and heavier, gaining layers like those of an onion. Eventually, it gets so heavy that it overcomes the updrafts of air in the cloud and falls to earth as the solid, icy hailstone we know. “On occasions,” says Atmosphere, Weather and Climate, “hailstones may reach giant size, weighing up to 0.76 kg [1.68 pounds] each.”

Did You Know?

On average, the water contained in the atmosphere all around the world is enough for only about ten days’ supply of rainfall.

One summer thunderstorm can expend as much energy as a dozen of the bombs that fell on Hiroshima during World War II. About 45,000 thunderstorms occur worldwide every day.

The atmosphere is not primarily heated by direct heat from the sun. Most of this heat energy passes straight through the atmosphere. It is heated by the energy that is radiated back into the atmosphere from the heated surface of the earth.

Water is the only abundant substance found on earth that exists simultaneously in the same locality in three different states—solid, liquid, and gas.

Fog is simply a cloud that forms at ground level.

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