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 agricultureindustryhygiene, 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 dustsaltsmoke, 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.

 

rain
Some animals avoid the rain. Other animals, like these Canada geese, have adapted to downpours.

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.

access
Noun

ability to use.

acid
Noun

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
Noun

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

Noun

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

air pressure
Noun

force pressed on an object by air or atmosphere.

animal rain
Noun

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

aquatic
Adjective

having to do with water.

Noun

an underground layer of rock or earth which holds groundwater.

Noun

layers of gases surrounding a planet or other celestial body.

Noun

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
Noun

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

condense
Verb

to turn from gas to liquid.

depression
Noun

indentation or dip in the landscape.

Noun

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

diameter
Noun

width of a circle.

drizzle
Noun

very light rain.

Noun

period of greatly reduced precipitation.

Noun

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

electrical energy
Noun

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

exhaust
Noun

gases and particles expelled from an engine.

fossil fuel
Noun

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

freshwater
Noun

water that is not salty.

hygiene
Noun

science and methods of keeping clean and healthy.

industry
Noun

activity that produces goods and services.

Noun

body of water surrounded by land.

landmass
Noun

large area of land.

meteorologist
Noun

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

methane
Noun

chemical compound that is the basic ingredient of natural gas.

microscopic
Adjective

very small.

minnow
Noun

very small fish.

nitrogen oxide
Noun

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

parachute
Noun

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

particle
Noun

small piece of material.

pollutant
Noun

chemical or other substance that harms a natural resource.

Noun

introduction of harmful materials into the environment.

Noun

all forms in which water falls to Earth from the atmosphere.

Noun

liquid precipitation.

rain dance
Noun

spiritual or ritual dance performed to bring rain.

raindrop
Noun

drop of liquid from the atmosphere.

Noun

area of tall, mostly evergreen trees and a high amount of rainfall.

Noun

large stream of flowing fresh water.

salt
Noun

(sodium chloride, NaCl) crystalline mineral often used as a seasoning or preservative for food.

saturate
Verb

to fill one substance with as much of another substance as it can take.

Saturn
Noun

sixth planet from the sun.

smoke
Noun

gases given off by a burning substance.

snow
Noun

precipitation made of ice crystals.

snowflake
Noun

precipitation that falls as an ice crystal.

spherical
Adjective

rounded and three-dimensional.

sulfur dioxide
Noun

greenhouse gas that can cause acid rain.

tadpole
Noun

stage in a frog or toad's development when the animal has gills and a tail, but not limbs.

Titan
Noun

largest moon of the planet Saturn.

tornado
Noun

a violently rotating column of air that forms at the bottom of a cloud and touches the ground.

tropical
Adjective

existing in the tropics, the latitudes between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.

volcanic eruption
Noun

activity that includes a discharge of gas, ash, or lava from a volcano.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

vortex
Noun

column of rotating fluid, such as air (wind) or water.

Noun

column of rotating cloud-filled wind that descends to an ocean or lake.