• rain
    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.

  • 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.

    Encyclopedic Entry: volcano
    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
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