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 * Section Review Answers**

Section: Water in the Air 1. Sample answer: Relative humidity is the amount of water vapor that the air ontains compared with the maximum amount of water vapor that the air can hold at a given temperature. Conden­ sation is a process that occurs when air reaches its saturation point. A cloud is a mass of air that contains millions of condensed water droplets. Precipitation is solid or liquid water that falls from a cloud. 2. C 3. If the amount of water vapor in the air increases, the relative humidity also increases. 4. The air contains 75% of the maximum amount of water that the air can hold at a given temperature. 5. Sample answer: Water evaporates from Earth’s surface and rises into the atmosphere. The air cools as it rises. As the air cools, water condenses and falls as precipitation. The precipitation falls to Earth. 6. rain, snow, sleet, and hail 7. Sample answer: Clouds that form at high altitudes are usually colder than clouds that form at lower altitudes. The cold, high-altitude clouds can be composed of ice crystals. 8. Sample answer: Cumulonimbus clouds can be very tall. Rain can form at the bottom of the cloud, and hail can form near the top of the cloud. 9. As the air temperature drops below the dew point, relative humidity increases to the saturation point and condensation occurs. 10. a cumulus cloud 11. Cumulus clouds form when warm, moist air rises. 12. Sample answer: Cumulus clouds usually indicate fair weather. However, when cumulus clouds grow large, they can become cumulonimbus clouds, which often produce thunderstorms.

Section: Air Masses and Fronts 1. Sample answer: An air mass is a large body of air that has a similar temperature and moisture level throughout. A front is a boundary between two air masses. 2. Sample answer: A cyclone is an area of low pressure with winds that spiral toward the center. An anticyclone is an area of high pressure with winds that flow outward. 3. A 4. continental polar, maritime polar, maritime tropical, and continental tropical 5. The North Pacific Ocean is one source region for maritime polar air masses. 6. A cP air mass is very cold and dry. It forms over areas near the poles, such as northern Canada.

7. The major types of fronts are cold fronts, warm fronts, stationary fronts, and occluded fronts. 8. Fronts are boundaries between air masses. When a front occurs, air masses of different temperatures and humidities interact. The interactions of air masses cause weather changes. 9. The rising air of a cyclone often causes stormy weather. The sinking air of an anticyclone often causes dry, clear weather. 10. 200 km 4 35 km/h 5 5.7 h 11. Sample answer: Air masses that form over land have less moisture than air masses that form over the ocean. The amount of moisture carried in both types of air masses affects the amount of precipitation that the United States receives. Air masses that form over the ocean generally bring precipitation. 12. Sample answer: The Pacific Coast has cool, wet winters because it is affected by a maritime polar air mass during the winter. Summers on the Pacific Coast are warm and dry because the weather is influenced by a dry continental air mass. 13. Answers may vary.