Download Chapter 6: AIR PRESSURE AND WINDS

Chapter 6: AIR PRESSURE AND WINDS
Answers to the Chapter Review
1.
Standard sea-level pressure equals 1013.25 millibars, 29.92 inches of mercury, and 14.7 pounds
per square inch.
2.
A glass tube filled with mercury is inverted into a dish of mercury. The mercury flows out of the
tube until the pressure at the base of the mercury column is balanced by the pressure exerted on
the surface of the mercury by the air above. When air pressure rises, the height of the column
increases and when air pressure drops, so does the height of the column. An aneroid (without
liquid) barometer consists of evacuated metal chambers that compress as air pressure increases,
and expand when pressure decreases. Advantages of aneroid barometers include that they can be
easily connected to a recording mechanism (a barograph) and that it can be used to indicate
altitude.
3.
Air pressure is the pressure exerted by the weight of air above. Air pressure decreases with an
increase in altitude because there is less air above to exert a downward force as one moves away
from Earth’s surface.
4.
Cold and dry air is more dense and therefore heavier than warm humid air. Since it is heavier, the
cold, dry air will tend to sink creating a high-pressure center. Because it is lighter, warm, humid
air will tend to rise and leave a low-pressure center.
5.
Convergence refers to air coming together. This occurs at surface low-pressure centers, where the
air then rises. The opposite, divergence, is air spreading apart. This occurs at surface high-pressure
centers. When divergence occurs aloft, this pulls air upward creating convergence on the ground,
or a surface low. Convergence aloft pushed air downward where it then diverges on the ground, a
surface high.
6.
Winds are only generated by horizontal pressure differences, that is, by the pressure gradient
force.
7.
Closely spaced isobars indicate a faster wind, while widely spaced isobars indicate a slower wind.
8.
After sunrise, the land heats more quickly and to a higher temperature than water. As the air over
the land warms, it expands, causing the isobars to bend upward. The pressure aloft is thus higher
than at comparable altitudes over the water. Therefore, the air aloft moves from the higher
pressure over the land toward the lower pressure over the water. The transfer of air aloft toward
the sea creates a surface high over the water and a surface low over the land. The surface
circulation that develops from this redistribution of mass aloft is from the sea to the land.
9.
Strong vertical currents are uncommon because the vertical pressure gradient is almost always
balanced by the opposing force of gravity.
10.
All free-moving objects, including the wind, are deflected to the right of their path of motion in
the Northern Hemisphere and to the left in the Southern Hemisphere.
1
11.
The two factors that influence the magnitude of the Coriolis effect are wind speed (the stronger the
wind, the greater the deflecting force) and latitude (strongest deflection at the poles and zero at the
equator).
12.
As soon as air begins to move from higher to lower pressure, the Coriolis force causes deflection.
As the air accelerates down the pressure gradient, the Coriolis force intensifies. The increased
wind speed results in even greater deflection. Eventually, the wind is turned so that it is flowing
parallel to the isobars, with the pressure gradient force directed toward the area of the lower
pressure and the opposing Coriolis force directed toward the area of higher pressure. When the
two opposing forces are equal, the wind will continue to flow parallel to the isobars at a constant
speed.
13.
Wind speeds near the surface are reduced by friction, while winds aloft are largely unaffected by
friction. Since the Coriolis force is proportional to wind speed, frictional drag at the surface
reduces the Coriolis force as it reduces the wind speed. Since the pressure-gradient force is not
affected by wind speed, it wins the “tug-of-war” against the Coriolis force. The result is the
movement of air at an angle across the isobars toward the area of lower pressure.
14.
See Figures 6-15 and 6-17.
15.
In order for a surface low to exist for a reasonable period of time, compensation must occur aloft.
For example, surface convergence would be maintained if divergence aloft occurred at a rate equal
to the inflow below.
16.
When the pressure tendency is rising, it means that a high-pressure center is approaching. Since
highs are associated with descending air, adiabatic warming precludes cloud formation. Thus,
clear fair weather is foretold by a rising barometer. Conversely, when the pressure tendency is
falling, a low-pressure center is approaching. Since lows are associated with rising air, cloud
formation is likely and precipitation is possible. Thus, cloudy and possibly rainy weather is
foretold by a falling barometer.
17.
When air moves from the relatively smooth ocean surface onto land, the increased friction causes
an abrupt reduction in wind speed. This reduction in wind speed downstream causes a pileup of air
upstream. Thus, converging winds and ascending air accompany flow off the ocean onto land.
Conversely, divergence and subsidence accompany the movement of air seaward because of
increasing wind speeds over the relatively smooth ocean surface.
18.
A southwest wind is blowing from the southwest toward the northeast.
19.
If the wind direction is 315, the wind is blowing from the northwest.
Answers to the Chapter Problems
1.
Assuming the air density at 5 kilometers is 0.75 kilogram per cubic meter, the magnitude of the
pressure-gradient force between the two cities is 0.186 cm/sec2.
2.
a. 0.084 cm/sec2
b. 0.132 cm/sec2
c. 0.125 cm/sec
2