Ch. 12 Questions - Moisture, Clouds, Precipitation

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Summarize the processes by which water changes from one state to another. Indicate whether heat energy is absorbed or liberated.

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).

Explain the principal of the sling psychrometer.

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, therefore little cooling of the wet-bulb. Thus, the closer together the wet and dry bulb temperatures are, the higher the relative humidity.

On a warm summer day when the the relative humidity is high , it may seem even warmer than the thermometer indicates. Why do we feel so uncomfortable on a "muggy" day?

8. Since the relative humidity is high, there would be a minimum of evaporation of perspiration, the body's natural cooling system.

Why does air cool when it rises through the atmosphere?

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

Why does the adiabatic rate of cooling change when condensation begins? Why is the wet adiabatic rate not a constant figure?

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

How do orographic lifting and frontal wedging act to force air to rise?

13. 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, thus the warm air is forced to rise over it. In both cases the rising air will cool adiabatically and cloud formation often results.

Explain why the Great Basin area of the western US is so dry. What term is applied to such a situation.

14. 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. To say the least, this deters cloud formation.

How does stable air differ from unstable air? Describe the general nature of the clouds and precipitation expected with each.

15. 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.

What is the function of condensation nuclei in cloud formation? What is the function of the dew point?

16. Condensation nuclei act as surfaces upon 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.

As you drink an ice-cold beverage on a warm day, the outside of the glass or bottle becomes wet. Explain.

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

What is the basis for the classification of clouds?

18. The primary basis for cloud classification is height (low, middle, high and clouds of vertical development) and form (cirrus, cumulus, and stratus).

Why are high clouds always thin?

19. The capacity of cold air for water vapor is very low. Since 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.

List five types of fog and discuss the details of their formation.

20. 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.

What is the difference between precipitation and condensation?

21. 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, 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.

List the forms of precipitation and the circumstances of their formation.

22. 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 size collide and coalesce. Atmospheric electricity is thought to be a key factor in this 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 upon impact with objects and the ground, creating an often-thick coating of ice.

Hail - 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. Layer after layer will collect and freeze as the hailstones are carried by updrafts above the freezing level.

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