Chapter 6: Air
Pressure and Winds
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Atmospheric pressure
Measuring air pressure
Surface and upper-air charts
Why the wind blows
Surface winds
Measuring and determining
winds
Atmospheric Pressure
We already know that air pressure is the weight
of the air above you
 We also know that if you move up in the
atmosphere, pressure
ALWAYS decreases
 But how does it change
in the horizontal?
For that, we need a
model
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Atmospheric Pressure
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Here, we have a model that comes with several
assumptions
Air molecules are NOT crowded close to the surface
 Width of air column does
NOT change with height
 The air is unable to move in
and out of the column
 What happens when we force
air into the column? Force air out
of the column? How do we do this?
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Atmospheric Pressure
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Now suppose we have two columns of air
with the same pressure, same
temperature, and same elevation
What happens when we cool one column
and warm the other? What happens to the
heights of the columns?
Atmospheric Pressure
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The cold column becomes more dense
and the warm column becomes less dense
The air pressure remains the
same/changes?
Atmospheric Pressure
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This means that a short column of air can exert
the same pressure as a tall column of warm air
Warm air aloft/high pressure. Cold air aloft/low
pressure
Atmospheric Pressure
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This pressure gradient causes a force (the
pressure gradient force) to move air from high
pressure to low pressure.
The pressure gradient force IS ALWAYS FROM
HIGH PRESSURE TO LOW PRESSURE
Atmospheric Pressure
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So, if we allow air movement, air will leak
out of column 2 and into column 1
This creates what kind of pressure at the
surface in each column?
Atmospheric Pressure
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So, we get a circulation where the air
above a high pressure is sinking and air
above a low pressure
is rising
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So, heating and cooling
can cause differences
in air pressure
Stepped Art
Fig. 6-2, p. 143
Barometers
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Barometer – an
instrument that measures
pressure changes
Mercury barometer
Barometers
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Aneroid barometer
Metal box called
aneroid cell
 Small changes in
pressure cause
cell to expand or
contract
 Different words on
readings display
weather types
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Pressure Readings
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Station pressure – the
reading from a
barometer not
corrected to sea-level
Sea-level pressure –
the reading from a
barometer corrected to
sea-level
 Adjustments are
usually 10 mb / 100
m
Pressure Readings
Pressure Readings
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Once all the
readings are
taken, you can
create isobars –
lines of constant
pressure
Specifically, this
is a surface map
Surface and Upper Air Charts
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Isobaric maps – shows height variations
along constant pressure
Surface and Upper Air Charts
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Contour lines - lines that connect equal elevation
above sea level
Isotherms – dashed red lines that show lines of
equal temperature (cold air to the north/lower
heights)
Surface and Upper Air Charts
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Ridges – elongated highs (warm air)
Troughs elongated lows (cold air)
Wind arrows blow parallel to lines in 500 mb
map
Why The Wind Blows
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Newton’s first law?
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An object at rest will remain at rest and an
object in motion will remain in motion
(constant velocity) as long as no force is
exerted on the object
Newton’s second law?
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Force exerted on an object equals its mass
times the acceleration. Acceleration is
change of speed OR DIRECTION
 F = MA
Forces that Influence the
Wind
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Wind is the net result of several forces. With
many forces acting, remember it is the net we
are looking for.
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Pressure gradient
force (PGF)
Coriolis force
Centripetal force
Friction
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Pressure Gradient Force
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Pressure gradient
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Difference in pressure / distance
Pressure gradient force – the force acting on air
that is directed from high pressure to low
pressure. The main reason for wind.
Strong PGF – sharp change in pressure with
distance
Weak PGF – gentle change in pressure with
distance
Fig. 6-11, p. 151
Coriolis Force
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Coriolis force – an apparent force due to
the rotation of the earth
Coriolis Force Movie
Coriolis Force
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While the ball moves in a straight path, the
merry-go-round rotates beneath it. By the
time gets to the other side, the receiver
has moved
It seems as though a force has acted on it,
but it is an apparent force because an
outside observer sees the ball go in a
straight path
This occurs for all free-moving objects
Coriolis Force
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Cause wind to deflect to the right in the
Northern Hemisphere and to the left in the
Southern Hemisphere
Coriolis Force
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Factors that affect the Coriolis Force (CF)
The higher the speed, the stronger the CF
 The higher the latitude, the stronger the CF
 CF is 0 at the equator and maximum at the
poles
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Straight-line Flow Aloft
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Example of PGF and CF in action
PGF always acts from
High to Low. CF
acts to the right of
motion
Geostrophic wind
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Wind that blows
parallel to isobars in a
straight path
Curved Winds Around Lows
and Highs Aloft
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Cyclonic flow – wind that blows around a low
Anticyclonic flow – wind that blows around a
high
Curved Winds Around Lows
and Highs Aloft
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Centripetal force – inward directed force in a
circular path
Gradient wind – wind that blows at a constant
speed, parallel to curved isobars
Curved Winds Around Lows
and Highs Aloft
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Thus, wind flows counterclockwise around a low
and clockwise around a high in the Northern
Hemisphere. Reverse for the Southern
Hemisphere
Winds on Upper-level Charts
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Where temperatures change quickly, large
height changes are noticed
Meridional – winds that blow north to south
Zonal – winds that blow west to east
Stepped Art
Fig. 6-19, p. 158
Surface Winds
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Planetary boundary layer (friction layer)
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Upwards to 1000 m.
Friction – interaction with surface that
decreases wind speed near the surface
Surface Winds
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Friction slows wind speeds, PGF becomes
greater than CF, and wind flows inward to
low pressure systems and outward from
high pressure systems
Winds and Vertical Motions
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Convergence into surface lows causes rising air
Divergence from surface highs causes sinking air
Winds and Vertical Motions
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Hydrostatic equilibrium – the balance between the
upward directed PGF and gravity
The Influence of Prevailing
Winds
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How do you determine wind direction?
Wind is given by the direction it is coming
from. Northerly wind blows from the north
Prevailing winds – wind most often
blowing during a given time
Wind Instruments
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Wind vane – points
into the wind
Anemometer –
measures wind
speed
Aerovane – gives
wind speed and
direction