Clouds are made up of millions of tiny droplets of water and ice. Each of these droplets of water is smaller than a grain of flour, and they are so light that they can float on air.

Three things need to happen for clouds to form: lifting (evaporation), cooling and condensation.

Air absorbs water by evaporation, so the air around us contains water vapor. When the air rises, it gets cooler and the water vapor condenses (turns into tiny drops of water) to form clouds. Condensation is the opposite of evaporation.

Clouds are pretty, but they can also tell you a lot about the weather. There are three main types of clouds, with lots of different variations.

Cirrus Clouds
Cirrus clouds are high in the sky. They are made from ice and look like thin, wispy white brush strokes against a blue background. They usually mean fair to pleasant weather. When you see cirrus clouds, it usually means the weather will change in the next 24 hours.

Stratus Clouds
Stratus clouds are grayish, and often cover the entire sky like a blanket. They look like fog that doesn't quite reach the ground. Sometimes, light mist or drizzle falls out of these clouds.

Cumulus Clouds
Cumulus clouds are white and puffy — they look like pieces of floating cotton or popcorn. They form during fair weather. The base of each cloud is flat and the top of each cloud is rounded. When the tops of the clouds start to look like cauliflower, it is called cumulus congestus or towering cumulus. These clouds grow upward and they can develop into giant cumulonimbus clouds, which are thunderstorm clouds.

Lenticular Clouds
These clouds look like UFOs! Because of their saucer-like shape, they have often been mistaken for spaceships. Lenticular (lens-shaped) clouds form at high altitudes, by strong winds forced over high mountains.