What is rain

Rain is liquid precipitation: water falling from the sky. Raindrops fall to Earth when clouds become saturated, or filled, with water droplets. Millions of water droplets bump into each other as they gather in a cloud. When a small water droplet bumps into a bigger one, it condenses, or combines, with the larger one. As this continues to happen, the droplet gets heavier and heavier. When the water droplet becomes too heavy to continue floating around in the cloud, it falls to the ground.

Human life depends on rain. Rain is the source of freshwater for many cultures where rivers, lakes, or aquifers are not easily accessible. Rain makes modern life possible by providing water for agriculture, industry, hygiene, and electrical energy. Governments, groups, and individuals collect rain for personal and public use.

Raindrops condense around microscopic pieces of material called cloud condensation nuclei (CCN). CCN can be particles of dust, salt, smoke, or pollution. Brightly colored CCN, such as red dust or green algae, can cause colored rain. Because CCN are so tiny, however, color is rarely visible.

When rain forms around certain types of pollutants, such as sulfur dioxide and nitrogen oxide, the CCN react with water to make the rain acidic. This is called acid rain. Acid can harm plants, aquatic animals like fish and frogs, and the soil. Sulfur dioxide and nitrogen oxide can be released into the atmosphere naturally, such as through a volcanic eruption. These pollutants can also be released by human activity, such as the burning of fossil fuels.

Burning fossil fuels can influence rain patterns. In urban areas, where many vehicles are on the road at once, rainfall is more likely during the weekend than during the week. This is because during the week, millions of cars release exhaust into the atmosphere, creating billions of CCN in the clouds. By the end of the week, clouds are much more likely to be saturated with moisture and CCN.

Scientists have developed a process called cloud seeding to "plant" CCNs in clouds to cause rain. Cloud seeding would reduce drought, although there is very little evidence that it works yet.

Although most people think raindrops look like teardrops, they actually look more like chocolate chip cookies. Like raw balls of dough dropped on a cookie sheet, the smallest raindrops, up to 1 millimeter in diameter, are actually spherical. At 2 millimeters raindrops start to flatten, because of the air pressure pushing up on them as they fall to Earth. This effect is increased at 3 millimeters, and depressions form on the bottom of the drops as the air pushes up on the drops harder. At 4 millimeters raindrops actually distort into a shape that looks like a parachute. When they get to be about 4.5 millimeters in diameter, raindrops are so big that they break apart into two or more separate drops.

Raindrops measure 0.5 millimeter (.02 inches) in diameter or larger. Drizzle, which is smaller than rain, consists of drops smaller than 0.5 millimeter. Most of Earth's precipitation falls as rain.

Raindrops often begin as snowflakes, but melt as they fall through the atmosphere. Snow forms in the same way rain does, but in colder conditions.

Rain falls at different rates in different parts of the world. Dry desert regions can get less than a centimeter (0.4 inches) of rain every year, while tropical rain forests receive more than a meter (3.2 feet). The world record for the most rain in a single year was recorded in Cherrapunji, India, in 1861, when 2,296 centimeters (905 inches) of rain fell.

FAST FACT

Animal Rain

It may not rain cats and dogs, but sometimes it rains tadpoles and tiny fish. This strange meteorological event is probably caused by waterspouts, basically tornadoes that form over water.

Waterspouts start out as vortexes, or columns of rotating, cloud-filled wind. As the vortex descends over an ocean or lake, small aquatic animals may be swept up in the waterspouts funnel.

Changes in pressure and wind force the waterspout to change back into a low-lying cloud, emptying precipitationincluding any creatures swept up in the waterspoutover a nearby landmass.

In 1894, newspapers in Bath, England, reported a rain of tadpoles. In 2009, a storm brought a rain of minnows down on Ishikawa, Japan.

Methane Rain

Rain forms on planets besides Earth. On Saturn's moon Titan, precipitation is not water, but methane. Titan received so much rain in 2009 that a new methane lake, four times as large as Yellowstone National Park, was formed.

Articles & Profiles

National Geographic News: Humans Changing Rainfall Patterns, Study SaysUSGS: Rain—A Valuable Resource.

access

ability to use.

acid

chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

acid rain

precipitation with high levels of nitric and sulfuric acids. Acid rain can be manmade or occur naturally.

agriculture

the art and science of cultivating land for growing crops (farming) or raising livestock (ranching).

air pressure

force pressed on an object by air or atmosphere.

animal rain

phenomenon where small aquatic organisms are swept up by a waterspout and fall as rain.

aquatic

having to do with water.

aquifer

an underground layer of rock or earth which holds groundwater.

atmosphere

layers of gases surrounding a planet or other celestial body.

cloud

visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

cloud condensation nuclei (CCN)

Plural Noun

microscopic bits of clay, salt, or solid pollutant around which water vapor condenses in clouds to form raindrops.

cloud seeding

process of adding chemical material to clouds in order to make it rain or otherwise control precipitation.

condense

to turn from gas to liquid.

depression

indentation or dip in the landscape.

desert

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

diameter

width of a circle.

drizzle

very light rain.

drought

period of greatly reduced precipitation.

dust

tiny, dry particles of material solid enough for wind to carry.

electrical energy

energy associated with the changes between atomic particles (electrons).

exhaust

gases and particles expelled from an engine.

fossil fuel

coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.

freshwater

water that is not salty.

hygiene

science and methods of keeping clean and healthy.

industry

activity that produces goods and services.

lake

body of water surrounded by land.

landmass

large area of land.

meteorologist

person who studies patterns and changes in Earth's atmosphere.

methane

chemical compound that is the basic ingredient of natural gas.

microscopic

very small.

minnow

very small fish.

nitrogen oxide

one of many chemical compounds made of different combinations of nitrogen and oxygen.

device which allows a person to glide down safely from a great elevation.