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
air pressure is measured with a barometer 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 What is air pressure? How does air pressure vary with elevation? Explain why humid air is lighter than dry air. How would you be able to tell where the windiest areas are on a weather map that shows isobars?