Geography 103

Geography 103
Weather
Lecture 6: Pressure and wind (Quiz 2)

1. Air pressure
    (1). Definition: The total weight of an air column on a given unit surface area ( 1 cm square, for example).
    (2). Pressure units:
          A. millibar (mb), inch (cm) mercury height, and hectopascal (hPa).
          B. 1 mb = 1 hPa = 0.0295 Hg (mercury height) = 0.0145 psi (pounds per square inch).
          C. Air pressure decreases with height at a given location
               (A). Standard sea level pressure = 1013 mb or 29.92 inches.
               (B). 18,000 ft or 5.5 km = 500 mb (middle atmosphere).
2. Wind (advection)
    (1). Definition: Horizontal movement of air current.
    (2). Wind direction (source of the wind, the direction from which the air comes)
           A. Compass directions:
                N, NE, E, SE, S, SW,W, NW.
                NE wind: The air comes from northeast and blows toward southwest.
           B. Azimuth angle
               (A). The clockwise angle between the north and wind direction.
               (B). North wind: 360^o or 36 (weather code)
                      Northeast wind: 45^o or 05
                      East wind: 90^o or 09
                      Calm (wind speed lower than 1 mph): 0^o or 00
    (3).Wind speed
          A. knot, mph (miles per hour), and m/s (meters per second)
          B. 1 knot = 1.15 mph = 0.5 m/s
3. Forces controlling winds
    (1). Pressure gradient force
          A. Pressure difference per unit distance.
          B. Triggers or starts the wind: air parcel moves from high to low pressure areas.
          C. Dense isobars (lines of equal air pressure): small isobar spacing.
    (2). Coriolis force
           A. Definition
                The deflecting force due to the rotation of the earth. The air parcel moves straight while
                the earth’s surface spins counterclockwise the Northern Hemisphere and clockwise in the
                Southern Hemisphere.
           B. Quantitatively, twice the spinning force around a local vertical axis (the axis through a point
                on the earth surface and the center of the earth).
          C. Coriolis force changes wind direction only but does not change wind speed.
          D. The magnitude of the Coriolis force is determined by wind speed (higher wind speed is
               accompanied by higher Coriolis force)
          E. Coriolis force is zero at the equator because the earth surface can not rotate around a local
              vertical axis at the equator.
          F. Maximum Coriolis force at the poles: Maximum rotation of the earth’s surface around the local
              vertical axis at the poles.
         G. Coriolis force deflects the wind direction to the right (look down wind or back to wind source) in
              the Northern Hemisphere.
   (3).Centripetal force: Force makes a curve motion (difference between pressure gradient force and
        Coriolis force)
        A. The inward force (points to the center of a curvature) that makes wind blow along a curve path.
        B. The difference between the pressure gradient force and the Coriolis force.
   (4). Friction force:
          A. Definition:
               A retarding force due to the unevenness of the earth surface (mountains, buildings, trees, etc.).
               Against the forward motion.
          B. Significant in the lower atmosphere or boundary layer (the air layer immediately above the ground
               surface where the friction force is significant, normally 1 km above the ground surface)
4. Types of winds
    (1). Friction wind or boundary-layer wind
          A. Lower atmosphere (boundary layer)
          B. Wind blows across isobars.
    (2).Gradient wind
          A. Upper-level wind: ignores the friction force.
          B. Curved isobars or contours.
          C. Wind blows more or less parallel to isobars or counters (more accurately tangent to isobars or
               counters).
          D. Involves centripetal force.
   (3). Geostrophic wind
          A. Upper-level wind: ignores the friction force.
          B. Straight isobars or contours.
          C. Absence of centripetal force.
5. Surface pressure systems (weather systems)
    (1). High pressure system (anticyclone or High)
          A. An area denoted by circular isobars with pressure increasing toward the center of the area.
          B. Clockwise circulation (motion of air) due to the Coriolis force in the Northern Hemisphere.
          C. Subsidence (sinking) and surface divergence (net horizontal outflow or net loss of air on
               a given area)
               (A). Speed divergence:
                      The upstream wind speed is lower than the downstream wind speed.
               (B). Directional divergence: Splitting of an air current or outflow of air.
          D. Warm and clear sky due to the compressional heating of the subsiding air.
          E. Fair weather and light wind.
    (2). Low pressure system (cyclone or storm)
          A. An area denoted by circular isobars with pressure decreasing toward the center of the area
          B. Counterclockwise circulation due to the Coriolis force in the Northern Hemisphere.
          C. Rising air (convection) and surface convergence (net horizontal inflow or net horizontal gain
               of air on a given area).
               (A). Speed convergence: The upstream wind speed exceeds the downstream wind speed.
               (B). Directional convergence: horizontal inflow of air.
           D. Cloudy or stormy weather due to the expansion cooling of air that produces clouds and possible
                precipitation.
6. Vertical wind variation (Ekman’s spiral): Wind direction veers (turns clockwise) with height and
    wind speed increases with height in the boundary layer.
7. Buys-Ballot law: Back to wind, the low pressure center is to the left.
8. Windchill index
    The equivalent air temperature at 3 mph wind for a set of measured air temperature and wind speed.
    The heat loss from the exposed skin is the same for the two sets of air temperature and wind speed
    combinations.