Precipitation is the falling of any form of water from the air to Earth’s surface. Precipitation occurs when cloud droplets grow into drops heavy enough to fall to Earth.
Raindrops form in two ways: by warm-cloud processes and by ice processes. In the warm-cloud process, tiny droplets form by condensation and then grow by bumping into and combining with other droplets. Droplets can collide because they are of different sizes. The bigger drops fall faster than the smaller ones. They catch up with smaller drops, collide with them, and capture them. Some smaller droplets get sucked behind the bigger drop and get captured that way. Still other droplets just bounce off the bigger ones.
Why do the droplets have different sizes? Droplets that have been in the cloud longer have had more time to “grow.” Some cloud droplets start larger because they have formed around a large salt nucleus. Still other cloud droplets get mixed into air that is less than saturated. The droplets shrink from evaporation. Mixing of air from different parts of the cloud or falling of larger dorps from higher up in the cloud brings droplets of different sizes together.
Droplets also grow by ice processes. Except for the shallowest clouds in the warm tropics, temperatures in the upper layers of clouds are below freezing. Both ice crystals and supercooled droplets are present. Supercooled water evaporates faster than ice, and this water vapor is deposited on the ice crystals. When the larger ice crystals get heavy enough, they start to fall. The falling crystals can then grow by capturing both the smaller ice crystals and water droplets in their paths. If the temperature in the lower part of the cloud is above freezing, the crystal melts and continues to grow by warm-cloud processes.
Precipitation comes in many forms. Drizzle consists of very fine drops that are very close together and fall very slowly. Raindrops are larger, farther apart, and fall much faster. A raindrop may have a maximum diameter of .25 of a centimeter. Larger raindrops may form, but they are torn apart as they fall.
Snow usually falls as clumps of six-sided crystals. The clumps grow by collision. When snowflakes fall into warm air, they partially melt into sticky, wet clusters. If the snowflakes melt completely, they fall as rain.
In some wintertime temperature inversions, warm clouds lie above a layer of air with a temperature below freezing. When raindrops fall through the freezing air, they turn into pellets of ice that fall to the ground as sleet. An ice storm, on the other hand, occurs when supercooled rain freezes instantly as it hits surfaces that are below freezing. Sheet ice, or glaze, forms on sidewalks, trees, roofs, and power lines. If the ice becomes heavy enough, trees and power lines may break under the weight of the ice.
Hail forms in cumulonimbus clouds. A hailstone begins as a frozen raindrop or small dense clump of ice crystals. A hailstone grows by collecting smaller ice particles, liquid cloud droplets, and supercooled raindrops that freeze onto it. The growing hailstone is kept aloft by a strong updraft until it becomes too heavy and falls out. In the early stages of its life, the hailstone may fall out of one updraft only to be carried aloft again in another. The size of the hailstone depends on how long it is kept aloft in the cloud and how much moisture it catches. Clearly, the stronger the updraft, the larger the hailstones can be.
Hailstones have a layered structure like an onion. This is because of the different temperatures and different kinds of moisture they meet on their trip through the thundercloud. Hailstones may be as small as 5 mm or as large as 140 mm.
The National Weather Service reports rainfall in hundredths of an inch. The rainfall is measured by and instrument called a rain gauge. The measurement represents the depth of water that the rain would leave if it did not soak into the ground, flow away, or evaporate.
Snowfall is measured in inches and tenths of an inch. A measurement is usually taken in an open location. The rain equivalent of the snowfall is determined by melting a definite depth of the snow. Dry snows are deeper than equal weights of wet snow. On the average, 10 inches of snow equal 1 inch of rain. This ratio, however, may range from as little as 5 inches of snow to as much as 30 inches of snow.
Precipitation – rain, sleet hail or snow – occurs in every part of the world. In some locations it may not rain for years at a time. In other places it may rain almost every day. Parts of Death Valley, California average only about 1 inch of rain a year. Cherrapunji in India averages 457 inches per year. What accounts for such differences in annual rainfall?
When air rises high enough and in large enough quantities, precipitation often occurs. The warmer the air, the more moisture it may contain. Also, the higher the air rises, the more moisture it can drop. It follows, therefore, that the rainy areas of the Earth will be those where air often rises in large quantities. Such areas are listed below.
The answer to this question is almost the exact opposite of “Where does it rain?” In areas of sinking air the air is warmed by compression. Thus no precipitation can occur. If sinking air persists, the dry conditions produce deserts. Sinking air occurs on the downwind or leeward side of mountains and in high-pressure areas. It also occurs where the global wind belts diverge, or blow away from a given area, at the surface. The only way air diverging at the surface can be replaced is by air sinking from above. This happens at the North and South Poles, and at the latitudes of the Sahara Desert.
Often it does not rain when or where it is needed the most. Ever since people have needed rain, they have tried to make it rain. An attempt to change the weather is called weather modification.
There are two methods of rainmaking. In the first method, very cold solid carbon dioxide, or “dry ice,” pellets are dropped into a supercooled cloud. These cool the cloud so much that tiny ice crystals form. The crystals grow by the ice process until they are heavy enough to fall. In the second method, artificial ice nuclei (usually crystals of silver iodide or lead iodide) are put into the cloud. Smoke generators are used to make the tiny crystals, which are very much like ice crystals in shape. Once ice crystals form, precipitation again grows by the ice process.
It is hard to prove that rainmaking works, since the precipitation that falls could have fallen naturally. Also, clouds must be present for rain to occur.
Scientists are not only searching for ways to make it rain. They are also trying to prevent hail and to eliminate fog at airports. Supercooled fogs can be cleared by the same techniques used to make rain. In this case, seeding the clouds makes the droplets grow too large and fall out.
Most acid rain contains nitrate or sulfate particles. Sulfates come from sulfur dioxide, which results from the burning of fuels and from natural sources such as volcanoes. Nitrate-forming gases come from those sources and also from automobiles.
Sulfate and nitrate particles make good condensation nuclei. When water condenses on them, they produce nitric or sulfuric acid. The drops that fall to the ground are known as acid rain. The dry sulfate and nitrate particles that fall to the ground are called dry acid deposition. They combine with groundwater to make acid.
Cloud droplets that form on these nuclei are very acidic. Acid clouds as well as acid rain heavily damage mountain forests because the high-altitude forests are often engulfed within the clouds. Not only does the soil become too acidic, but the leaves may be damaged by acid falling directly on them.
Acid rain is also destroying life in lakes and streams. Fish can no longer live in many lakes in the Adirondack Mountains of New York State because the water is too acidic. In cities, acid rain weathers rock and concrete and damages metals, paints, plastics, and paper.
