Encyclopedic Entry

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

Photograph by Jamie Lee, MyShot

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.

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.

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. Rain is up to 25 percent more likely on a Saturday than on a Monday in some urban areas.

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.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

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.

agriculture

Noun

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

Encyclopedic Entry: agriculture

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.

aquifer

Noun

an underground layer of rock or earth which holds groundwater.

Encyclopedic Entry: aquifer

atmosphere

Noun

layers of gases surrounding a planet or other celestial body.

Encyclopedic Entry: atmosphere

cloud

Noun

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

Encyclopedic Entry: cloud

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.

desert

Noun

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

Encyclopedic Entry: desert

diameter

Noun

width of a circle.

drizzle

Noun

very light rain.

drought

Noun

period of greatly reduced precipitation.

Encyclopedic Entry: drought

dust

Noun

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

Encyclopedic Entry: dust

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.

lake

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.

pollution

Noun

introduction of harmful materials into the environment.

Encyclopedic Entry: pollution

precipitation

Noun

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

Encyclopedic Entry: precipitation

rain

Noun

liquid precipitation.

Encyclopedic Entry: rain

rain dance

Noun

spiritual or ritual dance performed to bring rain.

raindrop

Noun

drop of liquid from the atmosphere.

rain forest

Noun

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

river

Noun

large stream of flowing fresh water.

Encyclopedic Entry: river

salt

Noun

mineral often used as a preservative or flavoring.

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.

urban area

Noun

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

Encyclopedic Entry: urban area

volcanic eruption

Noun

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

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.

waterspout

Noun

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

Encyclopedic Entry: waterspout

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writers

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrators

Tim Gunther
Mary Crooks, National Geographic Society

Editors

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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