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.

Variations

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.

cloud
We've looked at clouds from both sides now.

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.

Contrails
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.

Airavata
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.

accumulate
Verb

to gather or collect.

Noun

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

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.

Noun

layers of gases surrounding a planet or other celestial body.

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.

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.

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.

Noun

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

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.

Noun

overflow of a body of water onto land.

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.

Noun

sudden electrical discharge from clouds.

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.

Noun

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

plasma
Noun

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

Noun

introduction of harmful materials into the environment.

Noun

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

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

poisonous.

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.

Noun

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