3. Where does water in the atmosphere come from?

4. 3 Water Vapor Water vapor is an odorless, colorless gas. It mixes freely with other gases in the atmosphere. The amount of water vapor in the air varies from place to place and over time.

5. 4 Evaporation Water vapor enters the air through the process of evaporation. Evaporation is the changing of a liquid to a gas. Most of the water in the air evaporates from the oceans. Every day, millions of tons of water evaporate from the surface of the oceans. Water also evaporates from lakes, rivers, puddles, and wet soil. Winds carry the water vapor in the air all over Earth's surface.

6. 5 Near the equator, evaporation occurs rapidly.

7. How do clouds form?

8. 7 Cloud Formation From space, Earth sometimes seems to be covered with clouds. Clouds form from condensation in the atmosphere. Water droplets and ice form around dust and other particles in the air. Many billions of tiny water droplets and ice crystals form clouds. A cloud's shape is determined by how it formed.

9. 8 Kinds of Clouds Light, feathery clouds are called cirrus clouds. They are made up of ice crystals. They sometimes form at heights above 10,000 m.

10. 9 Cumulus Clouds Big, puffy, cottonball-like clouds with flat bases are called cumulus clouds. They form from rising currents of warm air that build to great heights. They are made of tiny water droplets.

11. 10 Stratus Clouds Sometimes the sky is covered with a layer of sheet-like clouds. These are stratus clouds. Stratus clouds form layer upon layer of water droplets, usually at low altitudes.

12. 11 Fog A cloud that forms near the ground is fog. Fog forms from condensation. At night, the ground cools quickly. It cools the layer of air that lies above it. The air may be cooled to the dew point. If it is, the water vapor condenses and forms fog.

13. 12 Thick blankets of fog can cover valleys or other low areas. Sometimes fog forms over rivers and lakes. This happens when cool air moves in over warm water.

14. 13 Cumulonimbus Clouds Cumulonimbus clouds are cumulus clouds that are often associated with thunderstorms. Updrafts can raise a cumulonimbus cloud to heights taller than Mount Everest. As the clouds rise, their tops become flattened against the tropopause, the top of the troposphere. This large, flat top resembles a blacksmith's anvil. An anvil is a good indication that a thunderstorm is coming.

15. 14 Cumulonimbus clouds are associated with thunderstorms.

16. 15 Checking Concepts 1.What are light, feathery clouds called? 2.What are sheet-like, layered clouds called? 3.What kinds of clouds are large and puffy?