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Two ingredients are necessary to form all clouds: water vapor and cloud condensation nuclei (CCN)--particles in the air such as dust or sea spray. If the air is saturated with water, the water vapor can condense into droplets or be deposited as ice crystals around CCN. A collection of billions of these tiny droplets forms a cloud.

A mass of air can become saturated with water when it is uplifted and cooled. Air can be uplifted by a number of different processes, including orographic ascent, convection, and convergence. Orographic ascent occurs when the shape of the landscape forces air upward, such as when the wind pushes air against a mountain. Convection occurs when air at ground level is heated by Earth's surface, becomes less dense, and then rises up through the cooler, denser air above. Air can also be pushed upward by the convergence of two air masses with differing temperatures. While most clouds are produced by uplift, some clouds are formed when water vapor is added to the air, for example, due to exhaust from an airplane.

Cloud properties vary depending on the temperature, air stability, and amount of CCN and water vapor in the atmosphere. Cloud names describe properties such as shape and altitude, as well as whether or not the cloud produces precipitation. For example, stratus clouds are layered clouds that uniformly cover large portions of the sky; cirrostratus clouds are high-level layered clouds composed of ice crystals; and nimbostratus clouds are low-level layered clouds that produce precipitation.

Precipitation is formed when cloud droplets or crystals grow too heavy to remain suspended in the atmosphere, and they fall to Earth's surface. Back on Earth, evaporation and transpiration change liquid water back into water vapor in the atmosphere.

In addition to playing a major role in the hydrologic cycle, clouds play an important role in helping to regulate the global climate. Low-level clouds reflect some of the Sun's radiation back into space, which has a cooling effect on Earth. At the same time, high-level clouds contribute to the greenhouse effect by acting like a blanket around Earth and absorbing some of the solar radiation or radiation from the surface of the Earth. A better understanding of cloud properties and formation will not only improve weather forecasting, but it will also help scientists predict global climate change.

• What are the major cloud types? How are they categorized?
• What clues help you identify the type of clouds at each level?
• What kinds of clouds are in the sky above your school now?
• What can clouds tell you about atmospheric conditions and the weather?
• Why do you think cumulonimbus clouds are associated with extreme weather such as thunderstorms and tornadoes?