Chapter 19: The Atmosphere in Motion 19.1 Air Pressure and Wind • What is Air Pressure? –The weight of the atmosphere as it pushes down upon Earth’s surface –Pressure is a force per unit area –Units: lbs/in2, millibars, hectopascals, inches of mercury –pressure decreases with height • What is Air Pressure? • air pressure is measured with a barometer –mercury barometers use mercury in a glass tube; the mercury rises and falls http://www.uwsp.edu/geo/faculty/ritter/images/atmosphere/pressure_wind/merc_barometer.jpg http://www.sciencemuseum.org.uk/images/object_images/535x535/10413056.jpg –aneroid barometers use a metal capsule filled with air that expands and contracts with air pressure changes http://www.free-online-private-pilot-ground-school.com/images/aneroid-barometer.gif http://www.trintec.com/images/Odyssey/ODY-04-Aneroid-Barometer-850.jpg • Why Does Air Pressure Change? –As temperature increases, pressure decreases • Molecules are farther apart in warmer air; closer together in cooler air –As humidity increases, pressure decreases • Water vapor weighs less than oxygen or nitrogen gas; pushes out these gases and exerts less force • Why Does Air Pressure Change? –Isobars: lines of equal pressure on a weather map –pressure is reduced to sea-level (removes elevation differences) to track high and low pressure systems –High pressure area = general good weather (clear, cool) –Low pressure area = general bad weather (cloudy or precipitation, warmer) • Why Does Air Pressure Change? –Pressure Gradient: pressure change over distance • isobars spaced closer together = windy • isobars spaced far apart = little wind http://www.srh.noaa.gov/jetstream/synoptic/wind.htm Weather Map http://www.hpc.ncep.noaa.gov/basicwx/92f.gif • What Makes the Wind Blow? –High pressure always moves toward low pressure to achieve balance –The greater the pressure difference, the stronger the winds –Ultimately caused by unequal heating of Earth’s surface • Measuring Wind –Wind vane: shows wind direction (a south wind comes from the south and moves toward the north) –Anemometer: measures wind speed http://www.peetbros.com/images/proawvsky.png • Review Questions 1. What is air pressure? How does air pressure vary with elevation? 2. Explain why humid air is lighter than dry air. 3. How would you be able to tell where the windiest areas are on a weather map that shows isobars?
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