Chapter 6

Air Pressure and Winds

1.    Newton’s Laws (forces) (pg 150)

·        First Law:  An object at rest will remain at rest, or remains moving in a straight line, unless acted upon by a force(s).

·        Second Law:  Acceleration of an object is proportional to the net force applied.

Force is proportional to acceleration

F prop. a

Force equals mass x acceleration

F = m x a

·        Note that if an object is changing direction (e.g., wind blowing around a low pressure system) then:

·        A force must be acting on it

·        It must be accelerating

2.    Four Forces Affecting the Winds

·        Pressure gradient force (PGF) (pg. 150)

·        Pressure gradient = pressure difference/distance

·        See Fig. 6.9, 6.10, 6.11

·        Coriolis force (effect) (pg. 151)

·        Arises from fact that earth is a rotating frame of reference, not stationary (Fig. 6.13, 6.14, 6.15)

·        To be significant, the air must be MOVING; it must NOT be on the equator (0° latitude); and it must be LARGE SCALE.

·        CF changes wind direction, not wind speed

·        Net result: winds deflected to the RIGHT in the N.H.

·        For a cool Coriolis video click here.

·        Centripetal force (derived from: center + towards) found where wind circulates around a Low or High.  It must exist because wind flowing around a curve (e.g., L or H) is accelerating (Newton’s second law) (see section 3, below)

·        Frictional force reduces wind speed due to surface roughness such as mountains, forests, etc. (see section 4, below)

3.    Upper Level Winds (here FF = 0 and can be ignored)

·        Wind speed and direction is due to balance of PGF (towards L), CF (to right of flow direction) and centripetal force (towards center of rotation (see Fig. 6.17, “Net” is due to centripetal force).

·        Geostrophic wind = straight line motion, parallel to contour lines

·        Net result of PGF and CF is a flow that is parallel to contours, NOT from high-to-low pressure, as one might have expected.

·        Low is on the left.

·        The stronger the PGF (steeper gradient) the stronger the wind (Fig. 6.16)

·        Rule of thumb – surface wind speed = 50% of 500 mb wind speed

·        Gradient wind = curved path around low/high pressure centers in upper atmosphere

·        Cyclonic flow = counterclockwise around a Low (in N.H.) (Fig. 6.17)

·        Anticyclonic flow = clockwise around a High (in N.H.) (Fig. 6.17)

·        Plotting upper level winds on constant pressure charts (Fig. 6.18; Figure 2, pg. 149; see also any 500 mb map)

·        Contour lines on all upper level charts represent altitude in decimeters (dm) and are spaced every 6 dm.  1 dm = 10 m.  Normal altitudes on a 500 mb chart are about 500 dm (= 5km).

·        Lowest altitudes correspond to coldest air and so are found in the north on the map (e.g., Canada, Alaska)

·        Troughs = cooler air = mid-latitude cyclones (pg. 148)

·        Ridges = warmer air = anticyclones (pg. 148)

·        Meridional flow = roughly N-S flow pattern (along the meridian lines)

·        Zonal flow = roughly E-W flow pattern (across time zones)

4.    Surface Winds (now FF must be included)

·        Friction force slows the wind in the “boundary layer” (Fig. 6.19).

·        Therefore, CF (which is proportional to wind speed) gets weaker.

·        If CF gets weaker then PGF begins to dominate and draws air across contour lines and into a Low (or pushes it away from a High).

·        Surface winds cross contours at an angle of about 30° and so they “fill in” a Low or they “empty” a High (Fig. 6.20).

·        Contour lines on all surface level charts represent altitude in millibars (mb), and are spaced every 4 mb.

·        Normal altitudes on a surface chart are about 1000 mb, and are all corrected to the mean sea-level pressure (MSLP) (pg. 147, Figure 6.7).

5.    Vertical Winds

·        Convergence and divergence (Fig. 6.21)

6.    Pressure Measurement (pg. 144)

·        Standard atmospheric pressure at sea level = 1013.25 mb (U.S. Standard Atmosphere, pg. 437); typical ranges from about 1050 mb to 880 mb (Figure 6.3)

·        Mercury barometer (Fig. 6.4)

·        Aneroid barometer (Fig. 6.5, 6.6, ASOS)

7.    Wind Measurement (pg. 160)

·        Wind speed – anemometer

·        Wind direction – wind vane.  Always name a wind after its origin (e.g., sea breeze originates from the sea; North wind originates from the North).

·        Rawinsonde (pressure, wind speed, wind direction with altitude; the skew-T diagram)

·        ASOS (Figure 6.26)

·        Doppler radar (from WSR 88-D radars, so-called NEXRAD) in cloudy air (pg. 136).