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Chapter 6: Air Pressure
and Winds



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Atmospheric pressure
Surface and upper-air
charts
Why the wind blows
Surface winds
Winds and vertical
air motions
Determining wind direction and speed
1
Atmospheric Pressure
air pressure at a given level is the weight of the air above
 air pressure and temperature
P = ρRT
(where R is a constant)
at constant P,
cold parcel is denser;
at constant T,
higher P means denser air;
at constant density, higher P means higher air T

Q: Because P = ρRT, higher T always leads to higher P
a) true, b) false
Q: When we say “warmer air parcel is less dense and hence
would rise”, the implicit assumption is
a) Parcel pressure is the same as the environment;
b) Parcel pressure is higher; c) parcel pressure is lower
2
Same
density
3
Q: Which statement is correct?
a) Warm air leads to high pressure in the mid-troposphere;
b) Cold air lead to high pressure in the mid-troposphere
Q: Which statement is correct?
a) It takes a shorter column of colder air to exert the same surface
pressure
b) It takes a taller column of colder air to exert the same surface
pressure
Q: Air flows from high pressure to low pressure at the same
altitude.
a) true, b) false
4
Measuring air pressure


mercury barometer
digital barometer in weather
observations
Standard atmospheric pressure:
1013.25 mb = 1013.25 hPa = 29.92 in.Hg
5
Pressure Readings
station pressure: surface P at specific location
if mercury barometer is used, corrections of
temperature, gravity, and instrument error (surface
tension of mercury) are needed
 sea-level pressure: obtained from station P with
corrections of altitude using
1 mb pressure increase for 10 m elevation decrease
 Isobars
constant pressure contour

Q: If 1 mb change corresponds to 10 m in height change
near surface, what would 1 mb change correspond to in
mid-troposphere? a) > 10 m, b) 10 m, c) < 10 m
6
Q: If surface pressure is 952 mb at 600 m above sea level,
its sea level pressure is: a) 892 mb, b) 952 mb, c)
1012mb, d) 1552 mb
Q: if surface pressure is 1032 mb at 100 m below sea
level, what is the sea level pressure?
Q: if sea level pressure is 1009 mb, what is the surface
pressure at 300 m above sea level?
7
Q: Can two
isobars drawn on
a surface weather
map ever
intersect?
a) yes,
b) no
8
Surface and Upper Air Charts


Surface map: isobars, high (H), low (L), cross-isobar flow
(note: sea level pressure is shown)
500 mb map: height contour lines, ridges, troughs,
flow parallel to height contours
(note: height above sea level at constant 500 mb is shown)
9
Q: Since the height at 500 mb is higher in the south than in the
north, the pressure in the south is: a) great than that in the north,
b) equal that in the north, c) less than that in the north
The thickness
between two
pressure levels (or
the height above
sea level at a given
pressure) is
proportional to the
average
temperature of this
layer: the higher
the temperature,
the greater the
height.
10
Q: Assuming pressure at point A is higher than that at B at the same
height (e.g., around 5500 m),
a) 500 mb height at A is greater than that at B;
b) 500 mb height at A is less than that at B;
c) 500 mb height at A is the same as that at B
Q: Assuming pressure at point A is higher than that at B at the same
height (e.g., around 5500 m), air temperature is
a) higher at A; b) higher at B; c) equal at A and B
Q: Why do height contours decrease
in value from south to north?
B
A
11
Why the Wind Blows
Newton’s first law of motion
An object at rest (or in motion) will remain at rest (or in
motion) as long as no force is exerted on the object
 Newton’s second law of motion
F = ma
(force = mass times the acceleration)
acceleration could be change of speed or direction

Four forces include pressure gradient force, Coriolis force,
centripetal force (or its opposite, centrifugal force), and
friction
Q: if F = 0, does the object still move?
a) yes, if it was moving;
b) no, if it was at rest;
c) both a) and b)
12
Forces that Influence the Wind

net force and fluid movement
• Wind is the result of a balance of several forces.
13
Pressure Gradient Force


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pressure gradient (pressure difference/distance)
pressure gradient force (PGF) (from high to low pressure)
strength and direction of the pressure gradient force
• The horizontal (rather than the vertical) pressure
gradient force is responsible for air movement.
Q: how to increase PGF?
a) increasing pressure
difference;
b) decreasing distance
between isobars;
c) both a) and b)
14
Q: where is the wind strongest in
the right figure (A, B, C, or D)?
A
B
D
A
C
Q: What is the wind
speed at point A?
a) 40 knots;
b) 40 miles/hour;
c) 40 km/hour
15
Coriolis Force

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Real and apparent forces
Coriolis force is an apparent force due to earth’s rotation
Its strength increases with the object’s speed, earth rotation,
and latitude (or more exactly
the sine function of latitude)
Its direction:
perpendicular to wind,
to the right-hand side over
Northern Hemisphere (NH),
and to the left over SH
Coriolis force changes the
direction only (but not the
wind magnitude)
16
Q: The claim that “water swirls down a bathtub drain in
opposite directions in the northern and southern hemispheres”
a) is true;
b) is false
Q: The Coriolis effect is stronger if
a) wind speed is faster;
b) latitude is higher;
c) both a) and b)
Q: What are
sin(30o)
and
sin(0o)?
17
Straight-line Flow Aloft

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balance of the pressure gradient
and Coriolis forces
geostrophic wind: parallel to
isobars with low pressure to its
left (or right) in NH (or SH)
good approximation for flow
aloft
• Geostrophic winds can be
observed by watching the
movement of clouds.
18
Curved Winds Around Lows and Highs Aloft
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cyclonic flow (with low P center) and anticyclonic flow (with
high P center): direction opposite in NH versus SH
clockwise and anticlockwise: same direction in NH and SH
centripetal force (opposite to centrifugal force)
gradient wind: balance of PGF, Coriolis and centrifugal forces
PGF > Co
Co > PGF
19
Q: what is the direction of PGF?
a) from high P to low P; b) from low P to high P;
c) depending on NH or SH
Q: what is the direction of Coriolis force?
a) to the right of movement in NH;
b) to the left of movement in NH;
c) to the right of movement in SH
Q: what is the direction of centrifugal force?
a) always outward;
b) always inward;
c) depending on NH or SH
Q: what is the balance of PGF, Co, and Centrifugal
forces for SH cyclonic flow?
a) PGF = Co + Cen; b) Co = PGF + Cen
20
Winds on Upper-level Charts

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meridional and zonal winds
wind is nearly parallel to the height contour
higher air T yields greater height contour value
• Height contours on upper-level charts are interpreted
in the same way as isobars on surface charts.
21
West wind over midlatitudes in NH and SH
Q: What is the wind
direction for a cyclone
over southern
hemisphere?
a) clockwise,
b) anticlockwise,
c) either way
22
Surface Winds
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planetary boundary layer: bottom 1 km above surface
Friction: opposite to wind in direction; increases with wind
frictional effects on the wind: slow down wind
Wind rotates clockwise from near surface to free atmosphere
in the NH
23
• Wind always moves cross isobars toward the low pressure center in
both NH and SH; it moves outward for the high pressure center.
• Wind rotates anticlockwise from near surface to free
atmosphere in the SH
24
Q: draw the three force (PGF, Co, Centrifugal) balance and
wind direction for a NH low pressure center.
Q: draw the three force (PGF, Co, Centrifugal) balance and
wind direction for a SH low pressure center.
Q: if surface wind is southwesterly in Tucson, the wind at
2000 m would be
a) southerly;
b) westerly;
c) southwesterly;
d) northeasterly
25
Winds and Vertical Motions


divergence and convergence (right-hand rule)
hydrostatic equilibrium (vertical PGF = gravity)
Q: Vertical PGF is much larger than horizontal PGF. a) true; b) false
Q: why does vertical PGF usually not result in upward motion?
26
Determining Wind Direction and Speed
wind direction: the direction where wind comes from
 prevailing wind: wind direction that occurs most frequently
 wind rose
Q: If the wind is southwesterly, the wind direction is
a) 45o;
b) 135o;
c) 225o;
d) 315o

27
Wind Instruments
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wind vane
cup anemometer
aerovane
rawinsonde
wind profiler
• By observing flags and smoke
plumes, our eyes are also
effective wind instruments.
Q: The arrow of the vane points
a) into the wind
b) away from the wind
28
Q: at 14:00 local time, the
near-surface wind is
a) westerly;
b) southerly;
c) southwesterly;
d) northeasterly
Wind
Power
29