Download Chap 6 air pressure winds

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Air-born dust from the Sahara Desert, Feb. 2001
Fig. 6-CO, p.140
FIGURE 6.1 A model of the atmosphere
where air density remains constant
with height. The air pressure at the
surface is related to the number of
molecules above. When air of the
same temperature is stuffed into the
column, the surface air pressure rises.
When air is removed from the column,
the surface pressure falls.
dust from China over
Japan.
3/5/2001
3/6/2001
Fig. 6-1, p.142
FIGURE 6.2 It takes a shorter column of cold air to exert the
same pressure as a taller column of warm air. Because of this
fact, aloft, cold air is associated with low pressure and warm
air with high pressure. The pressure differences aloft create a
force that causes the air to move from a region of higher
pressure toward a region of lower pressure. The removal of air
from column 2 causes its surface pressure to drop, whereas
the addition of air into column 1 causes its surface pressure to
rise. (The difference in height between the two columns is
greatly exaggerated.)
It takes a shorter column of cold, dense air to exert the same sfc P as a
taller column of warm, less dense air.
Warm air aloft = high P; cold air aloft = low P
Fig. 6-2, p.143
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Air pressure = force of air
molecules over a given area
The Pressure gradient force is
due to the pressure difference
and causes air to move from
higher P to lower P (wind)
Fig. 6-2c, p.143
FIGURE 6.3
Atmospheric
pressure in
inches of
mercury and
in millibars.
P=T x Density x k, so…
P~ T x P
Air above a region of surface high pressure is more dense than air
above a region of surface low pressure (at the same temperature
p.144
FIGURE 6.4 The mercury
barometer. The height of the
mercury column is a measure of
atmospheric pressure.
Fig. 6-3, p.145
Fig. 6-4, p.146
Recording barograph
Fig. 6-5, p.146
Fig. 6-6, p.147
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FIGURE 6.7 The top diagram (a) shows four cities (A, B, C, and D) at
varying elevations above sea level, all with different station pressures.
The middle diagram (b) represents sea-level pressures of the four cities
plotted on a sea-level chart. The bottom diagram (c) shows isobars drawn
on the chart (dark lines) at intervals of 4 millibars.
FIGURE 6.8 (a) Surface map (left) and
upper air map (right) for same day.
Fig. 6-7, p.147
FIGURE 6.8 (a) Surface map showing areas of high and low P
Wind blows across the isobars
Fig. 6-8a, p.148
Isobaric map
Because of the changes in air density, a surface of constant
pressure rises in warm, less-dense air and lowers in cold,
p.149
more-dense air.
Fig. 6-8, p.148
(b) Upper-level (500-mb) map for the same day on right.
Solid lines = contour lines in meters above sea level.
Dashed red lines = isotherms in °C. Note wind blows parallel
Fig. 6-8b, p.148
to the contour lines on upper air map!
FIGURE 6.9 The higher
water level creates higher
fluid pressure at the
bottom of tank A and a
net force directed toward
the lower fluid pressure
at the bottom of tank B.
This net force causes
water to move from
higher pressure toward
lower pressure.
Fig. 6-9, p.150
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P gradient = 4 mb/100 km
Net force = P gradient force = PGF
FIGURE 6.10 The pressure gradient between point 1 and point
2 is 4 mb per 100 km. The net force directed from higher
toward lower pressure is the pressure gradient force.Fig. 6-10, p.151
FIGURE 6.12 Surface weather map for 6 A.M . (CST), Tuesday, November 10, 1998. Dark gray lines are isobars with units in
millibars w/ 4 mb interval. A deep low with a central P of 972 mb (28.70 in.) is moving over NW Iowa. The dist. along the
green line X-X’ is 500 km. The diff. in P between X and X’ is 32 mb, producing a P gradient of 32 mb/500 km. The tightly
packed isobars along the green line are associated with strong northwesterly winds of 40 knots, with gusts even higher.
Wind directions are given by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (A wind indicated
by the symbol would be a wind from the northwest at 10 knots. See blue insert.) The solid blue line is a cold front, the solid
red line a warm front, and the solid purple (red ?) line an occluded front. The heav y dashed line is a trough.
Fig. 6-12, p.152
FIG. 6.11 The closer the spacing of isobars, the greater the pressure gradient. The
greater the pres. gradient, the stronger the pres. gradient force (PGF). The
stronger the PGF, the greater the wind speed. The red arrows represent the
relative magnitude of the force, which is always directed from higher toward lower
Fig. 6-11, p.151
pressure.
FIGURE 6.13 On non rotating platform A, the thrown ball
moves in a straight line. On platform B, which rotates
counterclockwise, the ball continues to move in a straight
line. However, platform B is rotating while the ball is in flight;
thus, to anyone on platform B, the ball appears to deflect to
the right of its intended path.
Fig. 6-13, p.152
geostrophic
wind
Coriolis effect
FIGURE 6.14 Except at the equator, a free-moving object will
appear from the Earth to deviate from its path as the Earth
rotates beneath it. The deviation (Coriolis “force”) is greatest
at the poles and decreases to zero at the equator.
Fig. 6-14a, p.153
FIGURE 6.15 Above the level of friction (~1000 m elev), air
initially at rest will accelerate until it flows parallel to the
isobars at a steady speed with the pressure gradient force
(PGF) balanced by the Coriolis force (CF).
Fig. 6-15, p.154
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FIGURE 6.16 The isobars and contours on an upper-level chart are like the banks
along a flowing stream. When they are widely spaced, the flow is weak; when they
are narrowly spaced, the flow is stronger. The increase in winds on the chart
results in a stronger Coriolis force (CF), which balances a larger pressure gradient
force (PGF).
Fig. 6-16a, p.155
FIGURE 6.17 Winds and related forces around areas of low
and high pressure above the friction level in the Northern
Hemisphere. Notice that the pressure gradient force (PGF) is
Fig. 6-17a, p.155
in red, while the Coriolis force (CF) is in blue.
FIGURE 6.18 An upper-level 500-mb map showing wind
direction, as indicated by lines that parallel the wind. Wind
speeds are indicated by barbs and flags. (See the blue insert.)
Solid gray lines are contours in meters above sea level.
Fig. 6-18, p.156
Dashed red lines are isotherms in °C.
Fig. 6-16b, p.155
FIGURE 6.17 Winds and related forces around areas of low
and high pressure above the friction level in the Northern
Hemisphere. Notice that the pressure gradient force (PGF) is
in red, while the Coriolis force (CF) is in blue.
Fig. 6-17b, p.155
Upper level chart that extends over the Northern and Southern
p.157
hemispheres. Solid gray lines on the chart are isobars.
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This drawing of a simplified upper level chart is based on
cloud observations. Upper level clouds moving from the
southwest (a) indicate isobars and winds aloft (b). When
extended horizontally, the upper-level chart appears as in (c),
where lower pressure is to the northwest and higher pressure
p.158
is to the southeast.
FIGURE 6.20 (a) Surf. weather map showing isobars and
winds on a day in Dec. in S. America. (b) The boxed area
shows the idealized flow around surf.-pressure systems in
Fig. 6-20, p.159
the Southern Hemisphere.
FIGURE 6.19 (a) The effect of surface friction is to
slow down the wind so that, near the ground, the
wind crosses the isobars and blows toward lower
pressure (friction lowers the wind speed which
lowers Coriolis force). (b) This phenomenon at the
surface produces an outflow of air around a high.
Aloft, the winds blow parallel to the lines, usually in
Fig. 6-19, p.159
a wavy west-to-east pattern.
FIGURE 6.21 Winds and air motions associated with surface
highs and lows in the Northern Hemisphere.
Fig. 6-21, p.160
FIGURE 6.22 An onshore wind blows from water to
land, whereas an offshore wind blows from land to
water.
Fig. 6-22, p.161
FIGURE 6.23 Wind direction can be expressed in degrees about
Fig. 6-23, p.161
a circle or as compass points.
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FIGURE 6.24 In the high country, trees standing unprotected
from the wind are often sculpted into “flag” trees. Fig. 6-24, p.161
FIGURE 6.26 A wind vane and a cup anemometer. These
instruments are part of the ASOS system. (For a complete
Fig. 6-26, p.162
picture of the system, see Fig. 3.17, p. 74).
FIGURE 6.25 This wind rose represents the percent of time
the wind blew from different directions at a given site during
the month of January for the past ten years. The prevailing
wind is NW and the wind direction of least frequency Fig.
is 6-25,
NE.p.162
FIGURE 6.27 The aero vane (skyvane).
Fig. 6-27, p.162
FIGURE 5 A portion of a wind farm near the summit of
Altamont Pass, California. With over 7000 wind turbines, this
is the world’s largest wind energy development project.
p.162
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