Chapter 17 Review Questions

1. Heat is absorbed when ice changes to water (melting), when water changes to water vapor (evaporation), and when ice turns directly to water vapor without passing through the liquid state (sublimation). Heat is liberated during condensation (the vapor-to-liquid phase change), freezing (the conversion of water to ice), and sublimation (the change from the vapor state directly to ice).

2. As temperature increases, the capacity of air to hold water vapor increases (at an increasing rate).

3. Relative humidity is expressed as a percent and indicates how close the air is to being saturated. Mixing ratio indicates the amount of water vapor in the air and is expressed as the mass of water vapor in a unit of air compared with the remaining mass of dry air.

4. (a) It is highest near sunrise and lowest during midafternoon.

(b) When the temperature was lowest and the relative humidity highest, that is, near sunrise.

5. The relative humidity will also decrease.

6. The mixing ratio is 2 grams per kilogram when the air temperature is -10°C and the relative humidity is 50 percent. When the relative humidity is 50 percent and the temperature is 20°C, the mixing ratio is 14 grams per kilogram.

7. Sling psychrometer: The lower the relative humidity, the more evaporation (and hence cooling) there will be from the wet-bulb thermometer. When relative humidity is high, there will be little evaporation and thus little cooling of the wet bulb. Thus the closer together the wet- and dry-bulb temperatures are, the higher the relative humidity. Hair hygrometer: the length of the hair changes as relative humidity changes.

8. Relative humidity = 62 percent; dew point = 9°C. If the wet-bulb temperature was 8°C, the relative humidity would drop to 29 percent, and the dew point would be –1°C.

9. Because the relative humidity is high, there will be a minimum of evaporation of perspiration, the body's

natural cooling system.

10. As air rises it expands because air pressure decreases with an increase in altitude. When air expands it cools adiabatically.

11. The environmental lapse rate refers to the temperature drop in the troposphere, that is, the temperature of the environment at different altitudes. It implies no air movement. Adiabatic cooling is associated only with ascending air that cools by expansion.

12. 18°C; Clouds would begin forming at 1000 meters.

13. The rate changes because of the addition of latent heat. Because the amount of condensation varies, so will the amount of latent heat released. Thus the wet adiabatic rate is variable.

14. The gases are expanding rapidly and cooling adiabatically.

15. Orographic lifting refers to mountains, which act as a barrier to the flow of air and thus force the air to ascend. Frontal wedging occurs when warm air glides over colder air. The cold air is denser, so the warm air is forced to rise over it. In both cases the rising air will cool adiabatically, and cloud formation will often result.

16. The Great Basin lies on the leeward side of the mountains. When air is forced to rise on the windward side of mountains, clouds and precipitation often occur (orographic uplift). However, when air descends on the leeward side, it warms by compression, thus deterring cloud formation.

17. Stable air resists upward movement, whereas unstable air does not. Clouds formed when stable air is forced to rise are generally thin, and precipitation, if any, is moderate or light. Conversely, when unstable air rises, clouds are often towering, and precipitation can be heavy.

18. During a temperature inversion the atmosphere is very stable, and the mixing depth is significantly restricted. Warm air overlying cooler air acts as a lid and prevents upward movement, leaving the pollutants trapped in a relatively narrow zone near the ground.

19. Condensation nuclei act as surfaces on which water vapor condenses. Saturated conditions are necessary for condensation and cloud formation. When the air is cooled to its dew point, the air is saturated.

20. The cold glass cools the air in contact with it below the air's dew point, and water vapor condenses on the glass.

21. Clouds are classified primarily by height (low, middle, high, and clouds of vertical development) and form (cirrus, cumulus, and stratus).

22. The capacity of cold air to hold water vapor is very low. Because temperatures above 6000 meters are very low, or air rising to these altitudes is very cold by the time it gets there, little water vapor is available for cloud formation.

23. Thunder—cumulonimbus; halos—cirrostratus; precipitation—nimbostratus and cumulonimbus; hail—cumulonimbus; mackerel sky—cirrocumulus; lightning—cumulonimbus; mare's tails—cirrus.

24. The first three fogs are formed when air is cooled below its dew point.

Advection fog forms when moist air moves over a cool surface.

Radiation fog forms on cool, clear, calm nights when radiation cooling is sufficient to bring air below its dew point.

Upslope fogs form by adiabatic cooling of air as it moves up a slope.

Evaporation fogs include steam fog and precipitation fog.

Steam fog forms when cool air overlies a relatively warm water surface. Often there is sufficient evaporation from the warm water to create saturation and fog.

Precipitation fog, as the name implies, forms when rain evaporates as it falls.

25. Condensation is the process by which water vapor changes to liquid water. Clouds and fog, the results of condensation in the atmosphere, are composed of very tiny water droplets that are too small to fall to the ground. Precipitation occurs when hundreds of thousands of cloud droplets coalesce into a drop large enough to fall to the ground.

26. Rain forms in supercooled clouds as ice crystals grow at the expense of cloud droplets. When the ice crystals grow large enough, they fall, often melting and reaching the ground as rain. If they do not melt, they reach the surface as snow. Rain also forms when cloud droplets of different sizes collide and coalesce. Atmospheric electricity is thought to be a key factor in the latter process.

Sleet results when rain falls through a subfreezing layer of air and freezes, reaching the ground as small pellets of ice.

Glaze occurs when rain falls through a relatively thin layer of subfreezing air. The drops become supercooled and freeze on impact with objects and the ground, creating an often-thick coating of ice.

Hail is composed of concentric layers of ice, is the largest form of precipitation, and is formed in cumulonimbus clouds. Here ice pellets grow by collecting supercooled droplets. Successive layers collect and freeze as the hailstones are carried by updrafts above the freezing level.

27. A trace of precipitation is an amount less than 0.025 centimeter (0.01 inch).

Answers to Earth System Questions

1. Although only a few hundred kilometers from the Pacific, the Great Basin desert of the western United States is situated on the eastern side of the imposing Sierra Nevada range, which acts as a moisture barrier for the region. Without the geologic evolution of the mountains to the west, the Great Basin region would not be located in a rainshadow and hence would have a much different climate.

Because precipitation is scanty, drainage is essentially interior, infiltration rates are often high, and the region has high evaporation,, no major rivers have their source in the Great Basin.

2. Although the pattern of monthly temperature changes for each place is similar, Flagstaff, at the higher elevation, has cooler temperatures and more precipitation owing to orographic lifting. As a consequence of these differences, vegetation surrounding Phoenix is typical of a dry desert, whereas that in Flagstaff is characteristic of the Colorado Plateau, with vegetation of broadleaf evergreen and dwarf shrub variety.

3. (a) An increase in elevation often brings more precipitation, especially on windward slopes.

(b) A decrease in the area covered by forests and other types of vegetation would reduce precipitation because less moisture would be added to the atmosphere from plants.

(c) The immediate response to lowering ocean-surface temperatures would be an increase in precipitation as global temperatures fell. However, eventually, the lower water temperatures would result in less evaporation of water to the air and a drop in precipitation.

(d) Winds blowing more frequently from an adjacent body of water would increase the amount of precipitation as more moisture was brought to the area.

(e) A major long-lasting episode of global volcanism would at first produce more precipitation as the atmosphere cooled. Eventually, less precipitation would fall owing to lower evaporation rates as a consequence of cooler temperatures.