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Chapter 8: Air Pressure and
Winds
Wind
What is wind?
What determines the direction of the wind?
What determines the wind speed?
How can we use weather charts to discern both wind
speed and direction?
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The answers have a lot to do with pressure variations!
Recall that the pressure decreases with altitude (a
reasonable approximation is 1 mb for every 10 meters)
But how does the pressure vary in the horizontal?
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In the horizontal, the
pressure varies only slightly,
i.e., only a few millibars over
several hundred
miles/kilometers.
Wind
In the horizontal, the
pressure varies only slightly,
i.e., only a few millibars over
several hundred
miles/kilometers.
It is these horizontal
variations in pressure that
drive winds.
Wind
In the horizontal, the
pressure varies only slightly,
i.e., only a few millibars over
several hundred
miles/kilometers.
It is these horizontal
variations in pressure that
drive winds.
Because these changes are
small, we have to remove
the effects of altitude to
better represent the
horizontal variability.
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Thus, to create the figure at
left, observed station
pressures are adjusted to
sea level, i.e., the pressure
that would be observed in
the station were located at
sea level.
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Correcting station
pressures allows a
surface pressure
chart (which is a
constant height
chart) to be
constructed.
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Example surface pressure chart
Joaquin
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Horizontal pressure differences can be generated by
changes in temperature
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The pressure
difference aloft,
pushes air toward the
left, which decreases
the pressure in the
second column,
forming a region of
low pressure at the
surface
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Recall the ideal gas
law:
p~rxT
For a constant
pressure, warming
means that the
density decreases!
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At the surface, we use a surface pressure chart, which is
a constant height chart, to depict pressure.
What about pressure variations aloft?
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We use constant pressure charts.
We showed how
pressure varies with
temperature; thus,
constant pressure
surfaces (e.g., the 500mb surface) slopes
down from warm to
cold air.
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The altitude at which
the given pressure is
found, is recorded on
a constant pressure
chart.
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The isoheight lines
display a wave-like
pattern with ridges
and troughs.
Ridges are
associated with air
that is warm.
Troughs are
associated with air
that is cool.
Wind: Quick Recap
Constant Height Chart (e.g., sea level chart): Locations with
the same pressure linked by isobars.
Constant Pressure Chart: Locations with the same height
linked by lines of constant height (isoheights).
Thus the constant pressure chart (isobaric chart) shows
height variations along an equal pressure surface.
At each location, the chart gives the height at which the
specified chart pressure is found.
Note that high heights are associated with warm air, and
vice versa.
To recap, constant height charts depict pressure
variations; constant pressure charts show height
variations.
Wind
Our consideration of atmospheric pressure at the
surface and aloft acts as a necessary preamble
to a discussion of the wind.
What causes the wind to blow?
Wind
Our consideration of atmospheric pressure at the
surface and aloft acts as a necessary preamble
to a discussion of the wind.
What causes the wind to blow?
Horizontal variations in pressure that create a
pressure gradient, which is indicated by a
change in pressure over a certain distance.
Wind
Our consideration of atmospheric pressure at the
surface and aloft acts as a necessary preamble
to a discussion of the wind.
What causes the wind to blow?
Horizontal variations in pressure that create a
pressure gradient, which is indicated by a
change in pressure over a certain distance.
The resulting force is called the pressure
gradient force.
Wind
We can relate forces to moving objects by
applying Newton’s second law, namely
Force = mass x acceleration (F = ma).
Thus, assuming an object’s mass is constant, an
exerted force is directly related to acceleration,
which the the change in velocity (speeding up,
slowing down, or changing direction).
Wind
Horizontal pressure
gradients result from
unequal heating of the
Earth’s surface.
The magnitude of the
pressure gradient can
be assessed by noting
the spacing of the
isobars. A wide isobar
spacing implies a small
pressure gradient, and
vice versa.
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The pressure gradient
force is directed inward,
towards the center of a
surface low (L).
The pressure gradient
force is directed
outward from the center
of a surface high (H).
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The magnitude of the pressure gradient force is
proportional to the pressure gradient itself
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Because the pressure gradient force acts at right angles
to the isobars (or isoheights), we might expect that winds
would also blow at right angles to the isobars (or
isoheights).
A glance at a
500 mb map
shows that
contrary to
expectation;
the winds blow
approximately
parallel to the
isoheight lines!
Wind
The winds aloft do not blow parallel to the isobars
and isoheights because of the action of the Coriolis
Force.
Properties of the Coriolis Force (CF)
1) The CF only occurs on Earth because it is rotating.
2) The CF always acts at right angles to the direction of
motion. (Right in NH)
3) The strength of the CF depends on the speed of the object
on which it is acting.
4) The strength of the CF also depends on the latitude. It is
equal to zero at the equator and is largest at the poles
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3) The strength of the CF depends on the speed of the object
on which it is acting.
4) The strength of the CF also depends on the latitude. It is
equal to zero at the equator and is largest at the poles
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Consider an air parcel initially at rest experiencing
a constant PGF above the surface in the NH
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When the parcel starts to move as a result of the
PGF, the CF increases in magnitude from zero and
deflects the parcel off to the right.
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As the parcel accelerates, the CF increases in
magnitude, pushing the parcel further off to the
right. Notice that the PGF remains constant.
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Eventually the parcel is deflected by the CF so much that it is
traveling parallel to the isobars, with equal pressure gradient and
Coriolis forces acting in opposite directions. The resulting
geostrophic wind blows parallel to the isobars.
Wind
Fun fact: Driving down a highway, your car would deviate to the
right by about 1500 feet for every 100 miles if it were not for the
friction between your tires and the road!
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It is the Coriolis force that causes air to move
counterclockwise (cyclonic) around low-pressure systems
and clockwise (anticyclonic) around high-pressure
systems.
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Winds on a nonrotating planet would
blow directly from
centers of high
pressure to centers of
low pressure
Winds on a rotating
planet like Earth, blow
around centers of high
and low pressure.
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Oh if only it were that simple . . . On top of the
PGF and CF, we have another force that we must
account for
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Friction with the surface slows the wind speed.
Slower wind speeds
reduce the magnitude
of the CF; thus the
deflection of air
parcels to their right is
reduced.
Consequently, the wind
no longer blows parallel
to the isobars, but
angles across them
toward lower pressure.
NH
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Example of flow patterns around high- and lowpressure systems
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It is this ageostrophyc component that gives rise
to convergence (divergence) at the centers of
low- (high-) pressure systems