Chapter 9 Section 1 What is the water cycle? Objective: Trace the steps in the water cycle

Key Terms evaporation: changing of liquid to a gas condensation: changing of a gas to a liquid precipitation: water that falls to Earth’s surface from the atmosphere water cycle: repeated pattern of water movement between Earth and the atmosphere

Evaporation and Condensation Liquids change to gases by a process called evaporation. Most of Earth’s surface is covered with water. When liquid water absorbs enough heat energy from the Sun, it changes into the gas water vapor. Air always contains some water vapor through evaporation. We become more aware of water vapor when the air is humid.

Evaporation and Condensation The changing of a gas to a liquid is called condensation. When air containing water vapor is cooled, the water vapor loses heat. If enough heat is lost, the water vapor changes to a liquid.

Evaporation and Condensation Water vapor condenses into tiny water droplets. These water droplets join to form clouds. If the temperature reaches freezing, ice forms. A cloud is a collection of water droplets or ice crystals.

1. DESCRIBE: What is a cloud?

Precipitation Water that falls to Earth from the atmosphere is called precipitation. Rain and snow are two forms of precipitation. As the water droplets in a cloud grow bigger, they become too heavy to stay in the air. Gravity pulls them toward Earth. The water falls as rain. If the rain passes through very cold air, the water may turn to a solid and fall to Earth as snow, hail, or sleet.

2. LIST: What are two forms of precipitation?

The Water Cycle Water is always changing state. As water evaporates from Earth’s surface, it changes from a liquid to water vapor. In the atmosphere, the water vapor condenses to a liquid. This forms clouds. Finally the water falls to Earth as precipitation. The repeated movement of water between Earth and the atmosphere is called the water cycle. The water cycle is also known as the hydrologic cycle.

The Water Cycle About 97 percent of Earth’s water is salt water. The rest is fresh water. About 76 percent of fresh water is frozen into ice caps near the North and South poles. A tiny amount is in the air in the form of water vapor. Only about 23 percent of Earth’s fresh water is readily available for use by living things. That water is found in lakes, rivers, and streams, and below Earth’s surface.

3. STATE: What is the water cycle?

Classwork CHECKING CONCEPTS What happens when water absorbs enough heat? During condensation, what happens to gas? What makes up a cloud? What processes make up the water cycle?

Classwork THINKING CRITICALLY 5. HYPOTHESIZE: How might the water cycle be affected when dust in the air blocks sunlight? 6. INTERPRET: Based on the fresh water figure, what would you say is the largest source of fresh water available for use by living things?