Download Chapter 4: MOISTURE AND ATMOSPHERIC STABILITY

Chapter 4: MOISTURE AND ATMOSPHERIC STABILITY
Answers to the Chapter Review
1.
Most water evaporates from the oceans. Some of the water vapor is transported considerable distances,
condenses to form clouds, and eventually falls to Earth’s surface as precipitation. Once precipitation
has fallen on the land, some of it reenters the atmosphere by evaporation and transpiration, some soaks
in, and some runs off. The water that soaks in or runs off eventually makes its way back to the ocean to
begin the cycle anew.
2.
This deficit is made up by runoff from the continents.
3.
Evaporation is the change of state from liquid to gas in which about 600 calories of latent heat per gram
of water are absorbed. The reverse process, condensation, releases about 600 calories per gram.
Melting is the change of state from solid to liquid. For each gram of ice that melts, 80 calories of latent
heat are absorbed. Freezing, the reverse of melting, liberates 80 calories of latent heat per gram.
Sublimation is the term used to describe the conversion of a solid directly to a gas without passing
through the liquid state. The term deposition is used to denote the reverse process. Sublimation and
deposition involve an amount of energy equal to the sum of the other two processes (680 calories).
4.
The higher the air temperature, the greater the amount of water vapor needed to saturate the air. As
temperature falls, so does the amount of water vapor needed for saturation.
5.
Mixing ratio and absolute humidity both indicate the actual amount of water vapor present in the air.
However, mixing ratio is stated as the mass of water vapor per unit mass of dry air, while absolute
humidity expresses the mass of water vapor per unit volume of air. Unlike mixing ratio and absolute
humidity, relative humidity does not state the actual amount of water vapor present in the air. Rather, it
is expressed as a percent and reveals how close the air is to being saturated.
6.
a. Just before sunrise; in midafternoon.
b. Just before sunrise.
c. If the amount of water vapor in the air remains constant, a rise in temperature will cause the relative
humidity to drop and a drop in temperature will cause the relative humidity to rise.
7.
The relative humidity decreases.
8.
A mass of air having a dew point temperature of 24 C contains four times more water vapor than air
having a dew point temperature of 4 C.
9.
Psychrometer: When air is passed over the wet bulb thermometer, some of the water will evaporate (the
amount of evaporation increases for lower relative humidities). Since evaporation is a cooling process,
the temperature of the wet bulb thermometer will fall. The magnitude of the temperature drop depends
upon how much evaporation occurs, which in turn depends upon the relative humidity. The greater the
temperature decline of the wet bulb thermometer (the bigger the difference between the wet and the dry
bulb readings), the lower the relative humidity.
Hair hygrometer: This instrument operates on the principle that hair (or certain synthetic fibers)
changes length in proportion to changes in the relative humidity, lengthening as relative humidity
increases and becoming shorter as relative humidity decreases.
10.
The hair hygrometer is less accurate than the psychrometer, requires frequent calibration, and is slow to
respond to changes in humidity. The advantage of the hair hygrometer is that it can be read directly
without using tables.
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11.
The process whereby the temperature of the air changes without the addition or subtraction of heat is
adiabatic temperature change.
12.
Unsaturated air cools at the dry adiabatic rate (1 C per 100 meters; 5.5 F per 1000 feet) when it rises
through the atmosphere.
13.
Air expands as it moves upward through the atmosphere because pressure decreases with increasing
elevation.
14.
Pressure increases as elevation decreases; thus, as air sinks it is compressed. When air is compressed, it
warms.
15.
When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced to the
wet adiabatic rate because the release of latent heat partially offsets the cooling due to expansion. The
wet adiabatic rate is not a constant figure because the amount of latent heat released depends on the
quantity of moisture present in the air.
16.
As the gas propellant in the can leaves the nozzle, it expands rapidly and thus cools adiabatically.
17.
Orographic lifting occurs when mountains or highlands act as barriers to the flow of air and force the air
to rise. Frontal wedging occurs when relatively warm (light) air rises over cooler (dense) air. The
cooler air acts as a barrier over which the warm air is wedged aloft.
18.
The Great Basin lies on the leeward side of large mountains. As air ascends the windward slopes of the
mountains, cloud formation and precipitation remove much of the water vapor. Farther on, as the air
descends the leeward slopes, it is warmed by compression, making condensation and precipitation
unlikely. Deserts such as the Great Basin are often termed rain shadow deserts.
19.
Localized convective lifting occurs because the air is warmer (less dense) than the surrounding air. This
is different from the other three processes that cause air to rise (orographic lifting, frontal wedging, and
convergence) because each of these force air to rise.
20.
Stable air resists vertical displacement, whereas unstable air reinforces vertical motion.
21.
The environmental lapse rate refers to the temperature change of the atmosphere as altitude increases. It
is variable and is determined by measurements (vertical soundings). Adiabatic cooling, on the other
hand, applies only to the cooling by expansion that occurs in vertically ascending air.
22.
The stability of the air is determined by its environmental lapse rate and its humidity. When the lapse
rate is less than the wet adiabatic rate, absolute stability prevails. When the lapse rate is greater than the
dry adiabatic rate, absolute instability prevails. Conditional instability exists when moist air has a lapse
rate between the dry and the wet adiabatic rates.
23.
Generally speaking, the lower the environmental lapse rate (the less air cools with increasing elevation),
the more stable the air is; conversely, the greater the environmental lapse rate, the less stable the air is.
24.
Towering cauliflower-shaped clouds, heavy precipitation, and isolated afternoon showers in summer
are conditions that are associated with unstable air.
25.
Instability can be enhanced by the following:
a) intense solar heating warming the lowermost layer of the atmosphere;
b) the heating of an air mass from below as it passes over a warm surface;
c) general upward movement of air caused by processes such as orographic lifting, frontal wedging, and
convergence;
d) radiation cooling from cloud tops.
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26.
Stability can be enhanced by the following:
a) radiation cooling of Earth’s surface after sunset;
b) the cooling of an air mass from below as it traverses a cold surface;
c) general subsidence within an air column.
Answers to the Chapter Problems
1.
a. 50%
b. 14.3%
c. 50%
d. It would increase from 50 to 100%.
e. 10 C
2.
53% and 12 C; 75% and 17 C
3.
15 C; 900 meters
4.
a.
b.
c.
d.
e.
f.
g.
h.
i.
5.
The temperature of water under an ice-covered lake must be greater than 0&degree; C. The maximum
density of water occurs at 4&degree; C, the water of this density and temperature will therefore sink to
the bottom of the lake.
1300 meters
0 C
0 C
about 4 grams per kilogram
35 C
about 35 grams per kilogram
about 22%
about 40%
the difference in temperature between points A and B; the air lost some moisture when traveling over
the mountains from point A to B
j. rainshadow desert
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