How do clouds form? Clouds have an important part in the water cycle. Clouds bring rain and snow to all parts of the world. Without clouds, rivers and lakes would become dry.

Temperature and Pressure  Clouds form when water vapor changes into tiny water droplets or ice crystals. Most diagrams of the water cycle include this change. It is a major part of the water cycle.  The temperature of air in the clouds is often much lower than the temperature of the air close to the ground.  Even on summer days, many clouds are made of ice crystals.

Temperature and Pressure  Air pressure affects the forming of clouds. Clouds often form when air moves upward to areas of less air pressure.  With less pressure, the air expands and cools. If the air cools enough at this new air pressure, water vapor will form droplets or ice crystals.

Review Question  What are clouds made of? Clouds are made of water droplets or ice crystals.

Types of Clouds Cirrus  High altitude clouds form more than 6,000 m above the ground. This region overlaps the region for mid-altitude clouds. Cirrus clouds are high-altitude clouds that are thin, wispy, and white.

Cumulonimbus  Clouds that grow vertically have rising air inside them. The bases of these clouds may be as low as 1,000 m above the ground. The rising air may push the tops of these clouds higher than 12,000 m up. Vertical clouds are sometimes called thunderheads because they often cause thunderstorms.

Altocumulus  The bases of mid-altitude clouds are between 2,000 m and 7,000 m above the ground. Altocumulus clouds are mid-altitude clouds that look like small, puffy balls. The bottoms of the clouds can look dark because sunlight may not reach them. The sides of the clouds are white because the sunlight is reflecting off them.

Stratus  Low-altitude clouds are often seen less than 2,000 m above the ground. Stratus clouds are low-altitude clouds that cover the whole sky. They look dark because little sunlight gets through the layer of clouds.

Fog  Fog is a cloud at ground level. One kind of fog can form on clear, cool nights with no wind. Air near the ground cools. If the air cools enough, water vapor condenses into tiny droplets and forms a cloud at or near the ground. As more droplets form and get larger, the fog appears thicker.

Review Questions What is a cloud at ground level? Fog Contrast low-altitude and high-altitude clouds. Low-altitude clouds can cover the whole sky and look dark because little sunlight gets through the layers. High-altitude clouds are thin, wispy, and white.

Why do some clouds form at low altitude and others form at higher altitude?  Air pressure, temperature, and water vapor affect the forming of clouds. Clouds form when water vapor condenses into water droplets or ice crystals or when air moves upward to areas of less air pressure and lower temperature. Sometimes this water vapor condenses close to Earth and sometimes it condenses at a higher altitude.

Precipitation  Most rain in the United States starts as snow.  The temperature of the air between the cloud and the ground determines whether precipitation will be rain, snow, or sleet.  A hailstone forms when a piece of ice is carried through a cloud many times. When the ice becomes too heavy for the winds to carry back up into the cloud, it falls to the ground as hail. Magic School Bus Kicks up a Storm Clouds

Review Questions What happens to snowflakes that fall through air that is warmer than 0 degrees Celsius? They melt and fall as rain. Why do snowflakes fall in the winter but not in the summer? In summer, the temperature of air near the ground is warmer than 0 degrees Celsius. Ice crystals that fall from clouds melt and become rain before they reach the ground. In winter, the temperature of air near the ground is colder. Thus, ice crystals may not get warm enough to melt.