• Clouds are visible accumulations of tiny water droplets or ice crystals in the Earth’s atmosphere. Clouds differ greatly in size, shape, and color. They can appear thin and wispy, or bulky and lumpy.

    Clouds usually appear white because the tiny water droplets inside them are tightly packed, reflecting most of the sunlight that hits them. White is how our eyes perceive all wavelengths of sunlight mixed together. When it’s about to rain, clouds darken because the water vapor is clumping together into raindrops, leaving larger spaces between drops of water. Less light is reflected. The rain cloud appears black or gray.

    Clouds form when air becomes saturated, or filled, with water vapor. Warm air can hold more water vapor than cold air, so lowering the temperature of an air mass is like squeezing a sponge. Clouds are the visible result of that squeeze of cooler, moist air. Moist air becomes cloudy with only slight cooling. With further cooling, the water or ice particles that make up the cloud can grow into bigger particles that fall to Earth as precipitation.

    Types of Clouds

    Because certain types of clouds are associated with certain types of weather, it is possible to forecast the weather by observing and understanding these different types of clouds.

    Clouds are classified into three main groups: cirrus, stratus, and cumulus.

    Cirrus clouds are wispy, curly, or stringy. They are found high in the atmosphere—typically higher than 6,000 meters (20,000 feet)—and are usually made of ice crystals. Cirrus clouds usually signal clear, fair weather. Their shape often indicates the direction the wind is blowing high in the atmosphere. 

    Stratus clouds are horizontal and stratified, or layered. Stratus clouds can blanket the entire sky in a single pattern. They usually occur close to the Earth. Stratus clouds often form at the boundary of a warm front, where warm, moist air is forced up over cold air. This movement produces clouds as the moist air is cooled across the entire front. The presence of stratus clouds usually means a chilly, overcast day. If precipitation falls from stratus clouds, it is usually in the form of drizzle or light snow.

    Cumulus clouds are large and lumpy. Their name comes from the Latin word meaning "heap" or "pile." They can stretch vertically into the atmosphere up to 12,000 meters (39,000 feet) high. Cumulus clouds are created by strong updrafts of warm, moist air. Most forms of heavy precipitation fall from cumulus clouds. The weather they bring depends on their height and size. The higher the base of a cloud is, the drier the atmosphere and the fairer the weather will be. Clouds located close to the ground mean heavy snow or rain.


    Clouds are also classified according to how high they are in the atmosphere and what kind of weather they produce.

    The prefix "cirro-" refers to clouds that lie more than 6,000 meters (20,000 feet) above the Earth. Cirrocumulus and cirrostratus clouds are two examples of these “high-level” clouds.

    The prefix "alto-" indicates clouds whose bases are between 2,000 and 6,000 meters (6,500-20,000 feet) above the Earth, such as altocumulus and altostratus clouds. They are considered "mid-level" clouds and are mostly made of liquid water droplets, but can have some ice crystals in cold enough temperatures.

    The prefix "nimbo-" or the suffix "-nimbus" are low-level clouds that have their bases below 2,000 meters (6,500 feet) above the Earth. Clouds that produce rain and snow fall into this category. ("Nimbus" comes from the Latin word for "rain.") Two examples are the nimbostratus or cumulonimbus clouds.

    Nimbostratus clouds bring continuous precipitation that can last for many hours. These low-level clouds are full of moisture.

    Cumulonimbus clouds are also called thunderheads. Thunderheads produce rain, thunder, and lightning. Many cumulonimbus clouds occur along cold fronts, where cool air is forced under warm air. They usually shrink as evening approaches, and moisture in the air evaporates. Cumulonimbus clouds gradually become stratocumulus clouds, which rarely produce rain.

    Clouds and Weather

    Certain types of clouds produce precipitation. Clouds also produce the bolt of electricity called lightning and the sound of thunder that accompanies it. Lightning is formed in a cloud when positively charged particles and negatively charged particles are separated, forming an electrical field. When the electrical field is strong enough, it discharges a superheated bolt of lightning to the Earth. Most of what we consider to be single lightning strikes are in fact three or four separate strokes of lightning.

    The sound of thunder is actually the sonic shock wave that comes when the air, heated by the lightning bolt, expands very rapidly. Thunder sometimes sounds like it comes in waves because of the time it takes the sound to travel. Because the speed of light is faster than the speed of sound, lightning will always appear before its thunder is heard.

    Meteorologists measure cloud cover, or the amount of the visible sky covered by clouds, in units called oktas. An okta estimates how many eighths of the sky (octo-) is covered in clouds. A clear sky is 0 oktas, while a totally overcast or gray sky is 8 oktas.

    Scientists have experimented with a process called cloud seeding for many years. Cloud seeding aims to influence weather patterns. Seeds, or microscopic particles, are placed in clouds. These seeds are artificial cloud condensation nuclei (CCN), which are tiny particles of dust, salt, or pollution that collect in all clouds. Every raindrop and snowflake contains a CCN. Water or ice droplets accumulate around CCN. Scientists hope that cloud seeding will allow people to control precipitation.

    Extraterrestrial Clouds

    Clouds exist in outer space. Clouds on Jupiter, for instance, are divided into three bands in the planet’s atmosphere. The highest band, at 50 kilometers above the surface of the planet, is mostly clear.

    Jupiter’s middle layer of clouds is constantly moving. These storm clouds appear as bands and swirls of yellow, brown, and red. Most of these clouds are made of droplets of ammonia and ammonia crystals, mixed with phosphorus and sulfur. (These ammonia storms would be toxic on Earth.)

    Beneath Jupiter’s thick layer of ammonia clouds lies what some astrophysicists believe is a thin layer of water clouds. Scientists think there may be water clouds because bursts of lightning have been spotted in Jupiter’s atmosphere.

    Interstellar clouds, which exist in the space between planets and stars, are not really clouds at all. Interstellar clouds are areas where gases and plasma are dense and, sometimes, visible. Astronomers determine what elements are present in interstellar clouds by analyzing the light, or radiation, that comes from them. Most interstellar clouds are made of hydrogen, helium, and oxygen. The dusty “milk” of the Milky Way is an interstellar cloud between the stars of our galaxy.

    We've looked at clouds from both sides now.

    Ancient Hindus believed the white elephant Airavata used his trunk to reach into the underworld and withdraw water. Airavata then sprayed this water into the sky, creating clouds and making precipitation possible.

    Contrails (short for condensation trails) are the linear clouds left behind a jet as it flies through the high atmosphere. These manufactured clouds result when the hot air expelled from the jets engine cools and condenses in the surrounding air.

    Internet Cloud
    "Cloud" is sometimes used as a metaphor for the Internet. The "cloud condensation nuclei" in the Internet cloud are websites around which users gather and contribute.

  • Term Part of Speech Definition Encyclopedic Entry
    accumulate Verb

    to gather or collect.

    air mass Noun

    a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

    Encyclopedic Entry: air mass
    ammonia Noun

    a gas (NH3) important to food production.

    ancient Adjective

    very old.

    astrophysicist Noun

    person who studies the relationship between matter, energy, motion, and force outside the Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    blanket Verb

    to cover entirely.

    characteristic Noun

    physical, cultural, or psychological feature of an organism, place, or object.

    cirrus Noun

    thin, high-altitude cloud.

    classify Verb

    to identify or arrange by specific type or characteristic.

    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 cover Noun

    amount of sky covered with clouds.

    cloud seeding Noun

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

    contrail Noun

    (condensation trail) linear cloud left behind as warm air ejected from a jet's engine cools.

    cumulonimbus Noun

    low-level cloud that produces rain, thunder, and lightning. Also called a thunderhead.

    cumulus Noun

    type of large cloud with a flat bottom and fluffy tops.

    dense Adjective

    having parts or molecules that are packed closely together.

    diameter Noun

    width of a circle.

    discharge Verb

    to eject or get rid of.

    drizzle Noun

    very light rain.

    dust Noun

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

    Encyclopedic Entry: dust
    electrical field Noun

    area that exists between particles with opposite (positive and negative) charges.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    engine Noun

    machine that converts energy into power or motion.

    evaporate Verb

    to change from a liquid to a gas or vapor.

    flood Noun

    overflow of a body of water onto land.

    Encyclopedic Entry: flood
    forecast Verb

    to predict, especially the weather.

    galaxy Noun

    collection of stars, planets, gases, and other celestial bodies bound together by gravity.

    gas Noun

    state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

    gradually Adverb

    slowly, or at a measured pace.

    helium Noun

    a light, colorless gas with the chemical symbol He.

    Hindu Noun

    religion of the Indian subcontinent with many different sub-types, most based around the idea of "daily morality."

    horizontal Adjective

    left-right direction or parallel to the Earth and the horizon.

    hydrogen Noun

    chemical element with the symbol H, whose most common isotope consists of a single electron and a single proton.

    ice crystal Noun

    solid ice arranged in precise molecular form.

    Internet cloud Noun

    cyberspace, or the network of connected facilities which store information from millions of computers connected to the internet.

    interstellar cloud Noun

    region of space where gas and plasma accumulate.

    jet Noun

    aircraft that moves by burning fuel.

    Jupiter Noun

    largest planet in the solar system, the fifth planet from the Sun.

    Latin Noun

    language of ancient Rome and the Roman Empire.

    lightning Noun

    sudden electrical discharge from clouds.

    Encyclopedic Entry: lightning
    metaphor Noun

    word or phrase used to represent something else, or an understanding of one concept in terms of another concept. 

    meteorologist Noun

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

    microscopic Adjective

    very small.

    Milky Way Noun

    galaxy in which the Earth and sun are located.

    nimbostratus Noun

    low-level cloud that produces continuous precipitation.

    okta Noun

    unit of measurement to determine how much of the sky, measured in eighths, is covered in clouds.

    overcast Adjective

    very cloudy.

    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.

    particle Noun

    small piece of material.

    phosphorus Noun

    chemical element with the symbol P.

    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    plasma Noun

    state of matter with no fixed shape and molecules separated into ions and electrons.

    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
    radiation Noun

    energy, emitted as waves or particles, radiating outward from a source.

    raindrop Noun

    drop of liquid from the atmosphere.

    reflect Verb

    to rebound or return light from a surface.

    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.

    shock wave Noun

    moving, measurable change in pressure and density of a material.

    snow Noun

    precipitation made of ice crystals.

    snowflake Noun

    precipitation that falls as an ice crystal.

    sonic Adjective

    having to do with sound or sound waves.

    star Noun

    large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

    stratify Verb

    to divide into layers.

    stratus Noun

    mid-level clouds found up to an elevation of about 2000 meters above the Earth.

    sulfur Noun

    chemical element with the symbol S.

    sunlight Noun

    visible radiation from the sun.

    thundercloud Noun

    cloud filled with electricity and capable of producing thunder and lightning.

    thunderhead Noun

    low-level cloud that produces rain, thunder, and lightning. Also called cumulonimbus.

    toxic Adjective


    underworld Noun

    mythical or legendary place for the souls of the dead.

    vapor Noun

    visible liquid suspended in the air, such as fog.

    vertical Noun

    up-down direction, or at a right angle to Earth and the horizon.

    warm front Noun

    mass of warm air that replaces a mass of cold air.

    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather