Encyclopedia of Science | Clouds

• Classification

Clouds are made up of minute water droplets or ice crystals that condense in the atmosphere. The creation of a cloud begins at ground level. As the Sun heats Earth's surface, the warmed ground heats the surrounding air, which then rises. This air contains variable amounts of water vapor that has evaporated from bodies of water and plants on Earth's surface. As the warmed ground-level air rises, it expands, cooling in the process. When the cooled air reaches a certain temperature, called the dew point, the water vapor in the air condenses into tiny microscopic droplets, forming a cloud. If condensation occurs below the freezing point (32°F; 0°C), ice crystals form the cloud. Clouds appear white because sunlight reflects off the water droplets. Thick clouds appear darker at the bottom because sunlight is partially blocked.

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