Chapter 6, Review Questions



Chapter 6, Review Questions

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1. Identify and describe the various phase change processes involving water.

a. Evaporation: water changes phase from liquid to vapor. Condensation: water changes phase from vapor to liquid. Deposition: water changes phase from vapor to solid without first becoming liquid. Sublimation: water changes phase from solid to vapor without first becoming liquid. Freezing: water changes phase from a liquid to a solid. Melting: water changes phase from a solid to a liquid.

2. Explain how the Sun drives the global water cycle.

a. Solar radiation from the Sun strikes the Earth’s surface and is absorbed (converted to heat). Some of this heat then evaporates water or sublimates snow and ice. The vaporization or sublimation of water causes clouds to form, which in turn causes precipitation. Solar radiation, therefore, powers the water cycle.

3. How does distillation convert seawater to fresh water?

a. As water evaporates (or sublimates) from the Earth, dissolved particles in seawater, such as salt, get left behind. This water vapor will then condense and eventually fall as fresh water.

4. Define vapor pressure. How does Dalton’s law of partial pressures apply to vapor pressure?

a. Vapor pressure is the total air pressure exerted by the water vapor component of air. Dalton’s law of partial pressures says that the total pressure of a gas is equal to the sum of the pressures of each constituent gas; therefore, when finding the total air pressure, water vapor’s contribution is known as the vapor pressure.

5. If the mixing ratio is equal to the saturation mixing ratio, what is the relative humidity?

a. When the actual concentration of water vapor in the air equals the water vapor concentration at saturation, the relative humidity is 100%.

6. On a clear calm day, why does the relative humidity usually decrease from a maximum shortly after sunrise to a minimum in the early or mid afternoon?

a. At constant vapor pressure, relative humidity varies inversely with the air temperature because the saturation vapor pressure increases with rising air temperature. As the air temperature rises from near sunrise to a maximum in the early to mid-afternoon, the saturation vapor pressure also increases. Because the air is calm, there is no advection and the actual vapor pressure remains essentially constant so that the relative humidity drops from near sunrise to early to mid-afternoon.

7. Why does the amount of precipitable water vary with the mean temperature of the troposphere?

a. The mean air temperature of the troposphere decreases as you move poleward, causing evaporation to decrease with an increase in latitude. This results in less precipitable water in polar regions. In general, warmer air has more precipitable water than colder air due to evaporation rates.

8. Describe the principal mechanism whereby clouds form in the atmosphere.

a. Water evaporates, and the air containing the water vapor ascends in the troposphere. As the air rises, pressure decreases, and the air expands and cools causing the relative humidity to increase. Once the relative humidity of the air reaches saturation, condensation occurs and a cloud forms.

9. Why are clouds and precipitation more likely on the windward slopes of a mountain range than the leeward slopes?

a. As air is forced to ascend the windward slopes of mountain ranges, it expands and cools resulting in an increase in relative humidity. The air continues to cool as it rises, eventually reaching saturation and producing clouds. When air descends the leeward slopes of a mountain range, it is compressed and warms. This causes its relative humidity to decrease, prohibiting clouds from forming.

10. What is the significance of a rain shadow for agriculture?

a. Rain shadows are regions of dry conditions on the leeward sides of mountain ranges. In these dry regions, an irrigation system may be used in order to grow crops. Only certain types of crops can be grown in these regions.

Chapter 6, Critical Thinking Questions

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1. Provide some examples of how the physical characteristics of Earth’s surface affect the ratio of how much rainwater (or snow melt) runs off versus how much infiltrates to the ground.

a. Topography greatly influences the amount of water that runs off. Mountainous terrain usually causes fast run off; the steeper the slope, the faster the water will run off. Water falling on flat land will more likely infiltrate to the ground. The physical properties of the land surface also affect how much water runs off or soaks into the ground. Rainwater (or snow melt) that falls onto sandy or unfrozen surfaces will infiltrate to the ground. However, water that falls onto frozen ground or city streets will likely run off. Vegetation can also intercept precipitation, as vegetated areas may intercept some precipitation before it reaches the ground.

2. How and why does the moisture content of the top soil affect the maximum air temperature in the lower troposphere?

a. As the moisture content of top soil increases, evaporation increases. Evaporation is a cooling process, therefore top soil with a high moisture content will keep temperatures cooler than dry soils would.

3. In late autumn or early winter, cold air flows on northwest winds from snow-covered ground across the ice-free waters of Lake Superior. Describe the changes in temperature, vapor pressure, and stability of the cold air as it flows over the lake.

a. The temperature of the water surface is higher than that of the snow-covered land surface. Hence, air that moves from snow-covered land to ice-free lake waters is heated from below causing the air temperature to warm and the stability to decrease. The water content of the air will increase due to increased evaporation as it moves over the lake, causing the vapor pressure to increase.

4. Are convective clouds (e.g., cumulus) more likely to form over snow-covered ground or bare ground? Explain your reasoning.

a. Convective clouds are more likely to form over bare ground than snow-covered ground. Air temperatures over snow-covered ground would be very low, inhibiting convection.

5. Determine the relative humidity if the…

{a} vapor pressure is 6 mb and the saturation vapor pressure is 24 mb.

{b}Vapor pressure is 12 mb and the saturation vapor pressure is 12 mb

{c}Mixing ratio is 10 g/kg and the temperature is 25°C.

a. {a}RH = [(vapor pressure)/(saturation vapor pressure)] x 100% = [(6 mb)/(24 mb)] x 100% = 25%, {b} RH = [(vapor pressure)/(saturation vapor pressure)] x 100% = [(12 mb)/(12 mb)] x 100% = 100 %, {c} Using Table 6.4 to find the saturation mixing ratio,

RH = [(mixing ratio)/(saturation mixing ratio)] x 100% = [(10 mb)/(20.44 mb)] x 100%

= 48.92%

6. Under what atmospheric condition is the actual air temperature, dewpoint, and wet-bulb temperature the same?

a. The air temperature, dewpoint, and wet-bulb temperature are equal when the air is saturated.

7. A saturated air parcel ascends in the atmosphere. What is the parcel’s initial relative humidity? What happens to the value of the relative humidity of the saturated parcel as it continues to ascend? What does this imply about the vapor pressure of the ascending saturated parcel?

a. If the parcel is initially saturated, its relative humidity is 100%. As the parcel ascends, its relative humidity will remain constant at 100%. If the relative humidity stays constant, so will the vapor pressure.

8. Determine whether the following soundings obtained from radiosonde launches are stable, unstable, or neutral for both saturated and unsaturated air parcels…

{a} -7 C°/1000 m

{b} +8 C°/1000 m

{c} -12 C°/1000 m

{d} -4 C°/ 1000 m

a. {a} Saturated: unstable, unsaturated: stable, {b} Saturated: stable, unsaturated: stable, {c} Saturated: unstable, unsaturated: unstable, {d} Saturated: stable, unsaturated: stable

9. How does stability affect the vertical development of cumuliform clouds?

a. Stable air suppresses vertical motion, whereas unstable air enhances vertical motion, convection, and expansional cooling. Hence, unstable air will favor the development of cumuliform clouds whereas stable air will suppress their development.

10. Explain how converging surface winds can cause clouds to form.

a. When surface winds converge, the air will be forced to rise. As the air rises, it will cool and expand causing the relative humidity to increase. Once the expanding air reaches saturation, clouds will form.

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