Chapter 16: Air Pressure and Wind

    I. Atmospheric pressure

       A. Force exerted by the weight of the air above

       B. Decreases with increasing altitude

       C. Units of measurement

             1. Millibar (mb) – standard sea level pressure is 1013.2 mb

             2. Inches of mercury – standard sea level pressure is 29.92 inches of mercury

       D. Instruments for measuring

             1. Barometer

                  a. Mercury barometer

                  b. Aneroid barometer

       II. Wind- Figure 16.3 and figure 16.6

       A. Horizontal movement of air

             1. Out of areas of high pressure

             2. Into areas of low pressure

       B. Controls of wind

             1. Pressure gradient force

                  a. Isobars – lines of equal air pressure-closely spaced-high winds

                  b. Pressure gradient – pressure change over distance

             2. Coriolis effect

                  a. Apparent deflection in the wind direction due to Earth's rotation

                  b. Deflection is

                      1. To the right in the Northern Hemisphere

                      2. To left in the Southern Hemisphere

       C. Upper air winds

             1. Generally blow parallel to isobars – called geostrophic winds

             2. Jet stream

                  a. "River" of air

                  b. High altitude

                  c. High velocity (120-240 kilometers per hour

III. Cyclones and anticyclones- Figure 16.9 pg. 449

       A. Cyclone

             1. A center of low pressure

             2. Pressure decreases toward the center

             3. Winds associated with

                  a. In the Northern Hemisphere

                      1. Inward (convergence)

                      2. Counterclockwise

                  b. In the Southern Hemisphere

                      1. Inward (convergence)

                      2. Clockwise

             4. Associated with rising air

             5. Often bring clouds and precipitation

       B. Anticyclone

             1. A center of high pressure

             2. Pressure increases toward the center

             3. Winds associated with

                  a. In the Northern Hemisphere

                      1. Outward (divergence)

                      2. Clockwise

                  b. In the Southern Hemisphere

                      1. Outward (divergence)

                      2. Counterclockwise

             4. Associated with subsiding air

             5. Usually bring "fair" weather

IV.  General atmospheric circulation

       A. Underlying cause is unequal surface heating

       B. On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat

       C. Idealized global circulation

             1. Equatorial low pressure zone

                  a. Rising air-Abundant precipitation

             2. Subtropical high pressure zone

                  a. Subsiding, stable, dry air

                  b. Near 30 degrees latitude

                  c. Location of great deserts

                  d. Air traveling equatorward from the subtropical high produces the trade winds

                  e. Air traveling poleward from the subtropical high produces the westerly winds

             3. Subpolar low pressure zone

                  a. Warm and cool winds interact

                  b. Polar front – an area of storms

             4. Polar high pressure zone

                  a. Cold, subsiding air

                  b. Air spreads equatorward and produces polar easterly winds

                  c. Polar easterlies collide with the westerlies along the polar front

      VI. Local winds

       A. Produced from temperature differences

       B. Small scale winds

       C. Types

             1. Land and sea breezes

             2. Mountain and valley breezes

             3. Chinook and Santa Ana winds

VII. Wind measurement

       A. Two basic measurements

             1. Direction

             2. Speed

      B. Direction

        1. Winds are labeled from where they originate (e.g., North wind – blows from the

             north toward the south)

             2. Instrument for measuring wind direction is the wind vane

       C. Speed – often measured with a cup anemometer

       D. Changes in wind direction

             1. Associated with locations of

                  a. Cyclones

                  b. Anticyclones

             2. Often bring changes in

                  a. Temperature

                  b. Moisture conditions