How is air pressure measured?
OBJECTIVE: Explain how a barometer measures air pressure. Key Term barometer (buh-RAHM-uht-uhr): instrument used to measure air pressure Lesson Summary There are two types of barometers: mercury and aneroid. A mercury barometer is a glass tube filled with mercury and open at one end. An aneroid barometer is an airtight metal container with very thin sides. Air pressure is measured in millimeters of mercury, or in units called millibars. CHAPTER 4 WEATHER 4-4 What is the dew point? OBJECTIVE: Explain what happens when air temperature goes above or below the dew point. Key Terms condensation (kahn-duhn-SAY-shuhn): changing of a gas to a liquid dew point: temperature to which air must be cooled to reach saturation frost: ice formed from condensation below the freezing point of water Lesson Summary When saturated air is cooled, condensation takes place; water vapor in the air condenses into droplets of water. Water vapor condenses at the temperature called the dew point. Dew forms when the
