Transcript Air Pressure and Winds

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
air pressure at a given level is the weight of the air above
 air pressure and temperature
P = ρRT
at constant P,
cold parcel is denser;
at constant T,
higher P means denser air;
at constant density, higher P means higher air T
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Q1: Because P = ρRT, higher T always leads to higher P
a) true, b) false
Q2: 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
Same density
Stepped Art
Fig. 6-2, p. 143
Q3: Which statement is correct?
a) Warm air leads to high pressure in the atmosphere;
b) Cold air lead to high pressure in the atmosphere
Q4: 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
Q5: Air flows from high pressure to low pressure at the same
altitude.
a) true, b) false
Measuring air pressure
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mercury barometer
digital barometer in weather
observations
Standard atmospheric pressure:
1013.25 mb = 1013.25 hPa = 29.92 in.Hg
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
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Q6: which statement is correct:
a) 1 mb change for >10 m height change for warm air;
b) 1 mb change for <10 m height change for warm air
Surface and Upper Air Charts
Surface map: isobars, high (H), low (L), cross-isobar flow
 500 mb map: height contour lines, ridges, troughs,
flow parallel to height contours
Q7: Why do height contours decrease in value from south to north?
a) because temperature is higher in the south;
b) because pressure is higher in the south
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Figure 2, p. 150
Q8: 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
Q9: Assuming 500 mb height at A is greater than that at B,
pressure at the same height (e.g., around 5500 m) would be
a) higher at A;
b) higher at B;
c) equal at A and B
Q10: 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
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
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Four forces include pressure gradient force, Coriolis force,
centripetal force (or its opposite, centrifugal force), and
friction
Q11: 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)
Forces that Influence the Wind
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net force and fluid movement
• Wind is the result of a balance of several forces.
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.
Q12: how to increase
PGF?
a) increasing pressure
difference;
b) decreasing distance
between isobars;
c) both a) and b)
Surface map
A
Q13: What is the
wind speed at point
A?
a) 40 knots;
b) 40 miles/hour;
c) 40 km/hour
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
Its direction:
perpendicular to wind,
to the right-hand side over
Northern Hemisphere (NH),
and to the left over SH
Q14: The claim that “water swirls down a bathtub drain in
opposite directions in the northern and southern hemispheres”
a) is true;
b) is false
Q15: The Coriolis effect is stronger if
a) wind speed is faster;
b) latitude is higher;
c) both a) and b)
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.
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 + Cen
Co = PGF + Cen
Q16: 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
Q17: 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
Q18: what is the direction of centrifugal force?
a) always outward;
b) always inward;
c) depending on NH or SH
Q19: what is the balance of PGF, Co, and Cen for SH
cyclonic flow?
a) PGF = Co + Cen; b) Co = PGF + Cen
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.
West wind over midlatitudes in NH and SH
Figure 4, p. 157
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
• 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
Fig. 6-21, p. 160
Q20: draw the three force (PGF, Co, Cen) balance and wind
direction for a NH low pressure center.
Q21: draw the three force (PGF, Co, Cen) balance and wind
direction for a SH low pressure center.
Q22: if surface wind is southwest in Tucson, the wind at
3000 m would be
a) southerly;
b) westerly;
c) southwesterly;
d) northeasterly
Winds and Vertical Motions
divergence and convergence
 hydrostatic equilibrium (vertical PGF = gravity)
Q23: Vertical PGF is much larger than horizontal PGF.
a) true;
b) false
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Measuring and Determining Winds
wind direction: the direction where wind comes from
 prevailing wind: wind direction that occurs most frequently
 wind rose
Q24: If the wind is southwesterly, the wind direction is
a) 45o;
b) 135o;
c) 225o;
d) 315o
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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.
Q25: at 14:00 local time,
the surface wind is
a) westerly;
b) southerly;
c) southwesterly;
d) northeasterly
Wind
Power
Fig. 6-29, p. 163