Clouds — visible collections of water in the air — are a regular feature of Earth's atmosphere. Depending on the conditions of the atmosphere, different types of clouds may form. For example, some clouds produce precipitation, such as rain or snow, while others do little more than appear as decorative figures in the sky.

Two ingredients are necessary to form all clouds: water vapor and 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 the particles. A collection of billions of these tiny droplets or ice crystals forms a cloud.

A mass of air can become saturated with water when it is uplifted and cooled. Air is uplifted by a number of different processes, including orographic ascent, convection, and convergence. Orographic ascent takes place when the shape of the landscape forces air upward; 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 it; and convergence happens when two air masses meet, forcing one of them upward. 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.

Clouds continuously shift and change shape because of air movement. They dissipate as the water droplets evaporate or move apart from each other. Winds also carry clouds across the sky. Because different levels of the atmosphere have different winds, it is possible to see clouds that are at different levels moving at different speeds.

If you watch a portion of the sky over a period of time, you will likely see clouds forming, moving, and changing shape. However, cloud movements are not always immediately obvious. Because the changes can be very slow, patience is needed to see the variations. With the help of time-lapse video, it is much easier to see changes in clouds over time.

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