October 21 2013 Chapter 8 Air Pressure and Winds Air Pressure The force exerted by the weight of air above a given point Pressure Units Commonly Used Milibar mb Hectopascal hPa Inches of mercury in Hg Millimeter of mercury mm Hg Standard Atmospheric Pressure at sea level 1013 25 mb 1013 25 hPa 29 92 in Hg 760 mm Hg More air molecules exert more force High Pressure Fewer air molecules exert less force Low Pressure Wind Pressure differences in the atmosphere result in air moving from high pressure to low pressure Pressure Measuring Instruments Barometers Hench the term Barometric Pressure 1 Mercury Barometer Invented by Torricelli Evangelista in 1643 Aneroid Barometers non liquid 1 Altimeter Calibrated to indicate altitude 2 Barograph Recording aneroid barometer Pressure changes with altitude decreases rapidly with altitude from surface first but decreases slowly with altitude aloft at roughly 18 400 ft 5 km pressure is about one half 500 mb of its sea level value Horizontal Pressure Variations Influenced by Air temperature Air moisture Air flow movement Forces Influencing Wind 1 Pressure Gradient Force Force due to differences in pressure within the atmosphere that causes air to move Shown on weather maps by spacing of isobars Isobars are lines on a map connecting places of equal air pressure Widely spaced isobars indicate gentle gradient lighter winds Closely spaced isobars indicate steeper gradient stronger winds When isobars are parallel winds flow almost at a right angle to the isobars 2 Coriolis Force 3 Friction Force