Self-study questions – Ch. 7

Water vapor saturation

The following questions are drawn from pp.167-176 :
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1.   Be able to able to explain the molecular basis for water vapor saturation and the meaning of “saturation vapor pressure” in this context.

2. It is not uncommon to hear someone say that air “holds” a certain amount of water vapor and that it can “hold” less vapor at lower temperatures.  Explain why air is not really relevant to the phenomenon of water vapor saturation.

3. Explain the relationship between the saturation vapor pressure of water, the boiling point of water, and atmospheric pressure.

4. Be able to correctly explain (and apply) the relationships between relative humidity, vapor pressure, dewpoint, and saturation vapor pressure, using either mathematical expressions or graphs of es(T).

Latent Heat

The following questions are drawn from pp.176-178 :
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1. Be able to explain the molecular basis for the latent heat of vaporization.

2. Be able to explain the meaning of, and relationship between, latent heats of (a) vaporization, (b) condensation, (c) fusion, (d) melting, (e) sublimation.

3. Given information about heat added or subtracted, be able to compute amounts or rates of melting, freezing, evaporation, etc.

Clausius-Clapeyron Equation

The following questions are drawn from pp.179-185 :
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1. What two approximations were made in order to derive (7.17)?

2. Given that (7.17) is an approximation, what determines where the formula is most accurate?

3. Where does (7.18) come from?  How were the coefficients A and B computed?  Determine new values that would ensure that (7.18) is most accurate in the vicinity of 40 degrees Celsius.

4. Equation (7.19) is called the Bolton formula.  How was it obtained?  Why is it sometimes preferable to the formula we derived from physical principles in (7.17)?

5. Based on the physical constants appearing in (7.17), what is the explanation for the different saturation vapor pressure with respect to water at a particular temperature vs. that with respect to ice?

6. Explain why the difference in saturation vapor pressure for ice and water, though small, is of great significance in the formation of precipitation.

7. Be able to use (7.18) to find saturation vapor pressure from temperature, vapor pressure from dewpoint, and vice versa.

Moisture variables

The following questions are drawn from pp.185-195 :
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1. Know how to use the relationship between saturation vapor pressure and saturation mixing ratio, as given by (7.23)

2. Know how to use saturation mixing ratio lines on a Skew-T diagram — relationship to temperature and/or dewpoint, behavior of parcels undergoing adiabatic ascent, etc.

3. Given the temperature, dewpoint (or dewpoint depression), and pressure, be able to determine the mixing ratio, saturation mixing ratio, and relative humidity from a Skew-T diagram.

4. Examining a sounding profile of temperature and dewpoint, be able to visually identify layers most likely associated with clouds.

5. From initial parcel properties (e.g., temperature, dewpoint, pressure or potential temperature and mixing ratio), be able to determine, and explain the significance of, the lifting condensation level (LCL).

Saturated adiabatic processes

The following questions are drawn from pp.195-209 and 222-227:
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1. Be able to explain the physics behind a moist adiabat.  In particular, why is the rate of change of temperature with decreasing pressure less than that for dry adiabatic ascent? 

2. How does one determine the equivalent potential temperature for a parcel using a Skew-T?  How about the wet-bulb temperature?  The wet-bulb potential temperature?  How are these three variables related?

3. Given two of the three variables \(T, T_w, T_d\),  use Normand’s rule to find the remaining one of the three.

4. Given that a parcel with specific properties is lifted to a new pressure level, be able to determine the adiabatic cloud water mixing ratio and/or density.

5. Be able to explain the difference between a reversible moist adiabatic process and an irreversible pseuodadiabatic process, with illustrations on a Skew-T of what happens on ascent and descent.

6. What parcel state variables are conserved for moist adiabatic and pseudoadiabatic processes?  What variable(s) is/are conserved only for moist adiabatic (reversible) processes?

7. Explain how an orographic cloud forms.

8. Be able to explain or reproduce the outcome of moist adiabatic and/or pseudoadiabatic processes as outlined on pp. 224-225.

9. Explain how cloud processes in the real world are or are not similar to the two idealized cases cited above.

10. Know which parcel variables are conserved under which conditions according to the table on p. 317.

Mixing processes

The following questions are drawn from pp.228-238:
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1. Identify the hallmarks of a well-mixed layer in the atmosphere.  In particular, which two variables are constant throughout the layer if it is unsaturated? What changes if part of the layer is saturated?

2. Explain why a well-mixed layer is often capped by an inversion.

3. Given the temperature and humidity properties of two different airmass and a plot of es(T), be able to determine under what conditions isobaric adiabatic mixing of the two air masses could lead to condensation. What are commonly observed examples of such mixing?