Clouds are visible accumulations of tiny water droplets or ice crystals in 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 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 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 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 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 zero oktas, while a totally overcast or gray sky is eight 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 (31 miles) 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.