October 21, 2013Chapter 8: Air Pressure and WindsAir 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 1643Aneroid Barometers: (non-liquid)1. Altimeter: -Calibrated to indicate altitude2. Barograph: -Recording aneroid barometerPressure 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 VariationsInfluenced by:-Air temperature-Air moisture-Air (flow) movementForces Influencing Wind1. 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 isobars2. Coriolis Force3. Friction