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.

• Cirrus Clouds

  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

  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

  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.