The Atmosphere in Motion:

1
The Atmosphere in Motion Air Pressure, Forces,
Winds

• This chapter discusses
• Measurement and meaning of surface and
  upper-level air pressure
• Effect of pressure and other forces on surface
  and upper-level winds

2
Atmospheric Pressure
Similar to earth's atmosphere, the pressure at
the base of this column of air results from the
weight of the gasses above. The earth's
atmosphere, however, has a greater density of
gases at its base due to gravity.
Figure 9.1
3
Temperature Elevation
When two columns of air are equal in elevation
and density, they are at equilibrium. Adjusting
temperatures by cooling (or heating) increases
(or decreases) air density. At the surface,
equilibrium is maintained, but the taller column
has greater upper-level pressure, and winds are
generated.
Figure 9.2A
4
Pressure Fluctuations
Figure 9.3
Solar heating of ozone gasses in the upper
atmosphere, and of water vapor in the lower
atmosphere, can trigger oscillating thermal tides
of sea-level pressure change.
5
Pressure Scale Units
Many scales are used to record atmospheric
pressure, including inches of mercury (Hg) and
millibars (mb). The National Weather Service
uses mb, but will convert to metric units of
hectopascals (hPa). The conversion is simply 1
hPa 1 mb.
Figure 9.4
6
Pressure Measurement
Figure 9.6
Changes in atmospheric pressure are detected by a
change in elevation of a barometric fluid or
change in diameter of an aneroid cell, which
indicates changing weather. Average sea level
pressure is 29.92 in Hg, or 1013.25 mb.
Figure 9.5
7
Pressure Trends
Figure 9.7
Barographs provide a plot of pressure with time,
and are useful in weather analysis and
forecasting. Altimeters convert pressure into
elevation, and are useful in steep terrain
navigation or flying. Both use aneroid cells.
8
Pressure Reading Reporting
Increase terrain elevation and decrease column of
air above. To remove the effect of elevation,
station pressure is readjusted to sea level
pressure at 10mb/100m. Isobars show geographic
trends in pressure, and are spaced at 4 mb
intervals.
Figure 9.8
9
Smoothed Isobar Maps
Continental maps of station recorded sea-level
pressure are often smoothed and simplified to
ease interpretation. Smoothing adds error to
those already introduced by error in instrument
accuracy.
Figure 9.9A
10
Constant Height Chart
Maps of atmospheric pressure, whether at sea
level or 3000 m above sea level, show variations
in pressure at that elevation.
Figure 9.10
11
Constant Pressure Chart
Maps of constant pressure provide another means
for viewing atmospheric dynamics. In this
example, there is no variation in elevation for a
pressure of 500 mb.
Figure 9.11
12
Variation in Height
Figure 9.13
Figure 9.12
Isobaric (constant pressure) surfaces rise and
fall in elevation with changes in air temperature
and density. A low 500 mb height indicates denser
air below, and less atmosphere and lower pressure
above. Contour lines indicate rates of pressure
change.
13
Ridges Troughs
Figure 9.14
Upper level areas with high pressure are named
ridges, and areas with low pressure are named
troughs. These elongated changes in the pressure
map appear as undulating waves.
14
Surface 500 mb Maps
Surface maps chart pressure contours, highs and
lows, and wind direction. Winds blow clockwise
around highs, called anticyclones. 500 mb maps
reveal patterns that on average are 5600 m above
the surface, where westerly winds rise and fall
across ridges and troughs.
Figure 9.15A
15
Forces Motion
Pressure forces are only one influence on the
movement of atmospheric air. Air responds
similarly as water to this force, moving from
higher pressure to lower pressure. Centripetal,
friction, and apparent Coriolis are other forces,
however, determining winds.
Figure 9.16
16
Pressure Gradient Force
Figure 9.17
Figure 9.18
Change in pressure per change in distance
determines the magnitude of the pressure gradient
force (PGF). Greater pressure changes across
shorter distances creates a larger PGF to
initiate movement of winds.
17
PGF vs. Cyclonic Winds
Pressure gradient force (PGF) winds acting alone
would head directly into low pressure. Surface
observations of winds, such as the cyclonic flow
around this low, reveal that PGF winds are
deflected by other forces.
Figure 9.19
18
Apparent Coriolis Force
Figure 9.20
Earth's rotation transforms straight line motion
into curved motion for an outside viewer. The
Coriolis force explains this apparent curvature
of winds to the right due to rotation. Its
magnitude increases with wind velocity and
earth's latitude.
Figure 9.21
19
Actual Observed Paths
Airplane travel paths have an apparent curvature,
just as Coriolis forces affect winds. Again, the
deflection between actual and observed paths is
greater near the poles.
Figure 9.22A
20
Geostrophic Wind
Figure 9.23
Winds have direction and magnitude, and can be
depicted by vectors. Observed wind vectors are
explained by balancing the pressure gradient
force and apparent Coriolis force. These upper
level geostrophic winds are parallel to pressure
contours.
21
Wind Speed Pressure Contours
Figure 9.24
Just as a river speeds and slows when its banks
narrow and expand, geostrophic winds blowing
within pressure contours speed as contour
intervals narrow, and slow as contour intervals
widen.
22
Isobars Wind Prediction
Upper level pressure maps, or isobars, enable
prediction of upper level wind direction and
speed.
Figure 9.25A
23
Centripetal Acceleration Cyclones
Acceleration is defined by a change in wind
direction or speed, and this occurs as winds
circle around lows (cyclones) and highs
(anticyclones). Centripetal force is the term
for the net force directing wind toward the
center of a low, and results from an imbalance
between the pressure gradient and Coriolis forces.
Figure 9.26A
24
Northern Southern Hemisphere Flow
Winds blow counterclockwise around low pressure
systems in the Northern Hemisphere, but clockwise
around lows in the Southern Hemisphere.
Figure 9.27A
25
Meridional Zonal Flow
Figure 9.28
Wind direction and speed are indicated by lines,
barbs, and flags, and appear as an archer's
arrow. Upper level winds that travel a
north-south path are meridional, and those
traveling a west-east path are zonal.
26
Friction Surface Winds
Surface objects create frictional resistance to
wind flow and slows the wind, diminishing the
Coriolis force and enhancing the effect of
pressure gradient forces. The result is surface
winds that cross isobars, blowing out from highs,
and in toward lows.
Figure 9.29A
27
Surface Flow at Lows Highs
Figure 9.30
Figure 9.31
Southern Hemisphere flow paths are opposite in
direction to Northern Hemisphere paths, but the
same principles and forces apply.
28
Sensing Highs Lows
The location of high and low pressure centers are
estimated by detecting surface wind direction and
noting pressure, Coriolis, and friction
forces. This figure illustrates the procedure
when standing aloft and at the surface.
Figure 9.32A
29
Vertical Air Motion
Figure 9.33
Winds converging into a low pressure center
generate upward winds that remove the
accumulating air molecules. These updrafts may
cause cloud formation. Likewise, diverging air
molecules from a high pressure area are
replenished by downward winds.