Chapt5 - Chapter 5 Atmospheric Pressure and Wind I II The...

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Chapter 5: Atmospheric Pressure and Wind I. The Impact of Pressure and Wind on the Landscape Pressure has an intimate relationship with wind: spatial variations in pressure are responsible for air movements II. The Nature of Atmospheric Pressure A. Intro 1. atmospheric pressure : force exerted by the gas molecules on some area of Earth’s surface or on any other body a. pressure is omnidirectional 2. atmospheric pressure at sea level = 14.7 pounds/in 2 a. atmospheric pressure decreases with increasing altitude B. Factors Influencing Atmospheric Pressure 1. pressure of a gas is proportional to its density and temperature 2. Density and Pressure a. density : mass of matter in a unit volume b. density of gas varies with location: a gas expands as far the environmental pressure will allow 1) pressure a gas exerts is proportional to its density 2) the denser the gas, the greater the pressure it exerts c. atmospheric pressure is directly proportional to the air density at that altitude 1) lower altitudes: atmospheric gas molecules are packed more densely because of stronger gravitational pull more molecular collisions higher pressure 2) higher altitudes: less dense air decrease in pressure 3. Temperature and Pressure a. heated air increase molecular agitation increase speed greater collision force higher pressure b. if other conditions remain the same 1) an increase in gas temperature increase in pressure 2) a decrease in temperature decrease in pressure c. when air is heated and no control kept on volume, it expands decreased density 1) warm weather associated with low atmospheric pressure 2) cool weather associated with high atmospheric pressure 4. Dynamic Influences on Air Pressure a. intricate relationship between atmospheric pressure, air density, and air temperature b. generalizations about surface pressure : 1) very warm surface conditions often produce low pressure at the surface – a thermal low 2) strongly rising air often produces low pressure at the surface dynamic low 3) very cold surface conditions often produce high pressure at the surface – thermal high
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Chapter 5: Atmospheric Pressure and Wind – p. 2 of 10 4) strongly descending air often produces high pressure at the surface – a dynamic high C. Mapping Pressure with Isobars 1. barometer : instrument used to measure atmospheric pressure 2. millibars (mb): unit of measure of atmospheric pressure a. average sea level pressure = 1013.25 mb (29.92 in) 3. isobar : isolines of equal pressure 4. pressure is relative a. pressure is higher or lower than that of surrounding areas b. roughly circular or oval isobars on weather map indicates an area of high or low pressure c. a ridge of high pressure: separates 2 isobars of low pressure; a trough of low pressure: separates 2 isobars of high pressure 5. air pressure readings usually adjusted to represent pressures at sea level 6. pressure gradient : horizontal rate of pressure change a. relative closeness of isobars indicates the pressure gradient b. closely spaced isobars indicate
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