Key to Exam 1 Version 1

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Key to Exam 1 Version 1 Powered By Docstoc
					                                                                       Dr. Droegemeier

Student ID ___________________________________

                                     Meteorology 2603
                                Severe and Unusual Weather
                                        Spring, 2001

             Solutions to Examination #1: Test Version = 1
                     Wednesday, 21 February 2001
BEFORE YOU BEGIN!! Please be sure to read each question CAREFULLY and, if time
permits, check your answers. Answer all questions on the pages provided - do not use
additional sheets. Be sure to submit your computer form as well as the exam before leaving
the room.

Part I: Multiple Choice (2 points each) - Indicate the correct answer on the computer form
using a #2 pencil.

1. Which of the following is NOT a unit of pressure

a.   Millibars
b.   Bars
c.   Inches of Mercury
d.   Pascals
e.   Kilograms

2. The ideal gas law relates the following quantities to one another
a.   Pressure, wind, and humidity
b.   Pressure, temperature, and humidity
c.   Pressure, density, and humidity
d.   Pressure, temperature, and density
e.   None of the above

3. The conversion of ice directly into water vapor

a.   Releases thermal energy
b.   Requires thermal energy
c.   Is known as sublimation
d.   Is not physically possible without first going through the melting process
e.   Both b and c
4. Single cell thunderstorms (also called air mass thunderstorms) most often form in
environments characterized by

a.   Strong stability and weak winds
b.   Strong instability and strong winds
c.   Weak instability and strong winds
d.   Weak instability and weak winds
e.   None of the above

5. Which of the following is NOT typically associated with a thunderstorm?

a.   Cirrus clouds
b.   Hail
c.   Heavy rain
d.   Lightning
e.   Stability
6. Which type of thunderstorm always produces severe weather?

a.   Single cell
b.   Super cell
c.   Multicell
d.   Squall line
e.   Macro cell

7. Air cools when it rises because

a.   The temperature of the environment decreases with height
b.   Water within the parcel evaporates
c.   The pressure of the environment decreases with height
d.   Both a and c
e.   a, b, and c

8. Suppose two columns of air have identical depth. The column of air having a greater density

a.   Have a higher surface pressure
b.   Have a higher surface temperature
c.   Have more air molecules per cubic centimeter
d.   Have the same number of air molecules per cubic centimeter
e.   Both a and c.

9. The MKS system of units involves the use of

a.   Meters, degrees Kelvin, and pounds
b.   Meters, kilograms, and seconds
c.   Meters, kilograms, and slugs
d.   Meters, kilometers, and seconds
e.   None of the above
10. Dynamic pressure is caused by

a.   The force of the wind
b.   The weight of the air above you
c.   The area over which the force acts
d.   Changes in density
e.   None of the above

11. If an air parcel becomes saturated during its ascent, its rate of cooling with height

a.   Decreases
b.   Stays the same
c.   Increases
d.   Isn't affected
e.   None of the above
12. The stratopause marks the transition between the

a.   Troposphere and stratosphere
b.   Stratosphere and mesosphere
c.   Stratosphere and thermosphere
d.   Troposphere and ionosphere
e.   Mesosphere and thermosphere

13. Radiant energy can be transferred by

a.   Conduction
b.   Convection
c.   Dilution
d.   Dissipation
e.   Both a and b

14. Air accelerates from high to low pressure because of

a.   The centrifugal force
b.   Changes in temperature
c.   Adiabatic cooling
d.   Latent heat release
e.   The pressure gradient force

15. The moist (also called saturated) adiabatic lapse rate is approximately

a.   11 deg C/km
b.   10 deg C/km
c.   6.5 deg C/km
d.   12 deg C/km
e.   5 deg C/km
16. In a stable atmosphere, the temperature of a rising parcel of air decreases

a.   Faster than the environment
b.   Slower than the environment
c.   At the same rate as the environment
d.   The parcel temperature doesn't change
e.   None of the above

17. Energy is

a.   The capacity to do work
b.   Stored by the atmosphere in the form of water vapor
c.   Released or absorbed when water changes phase
d.   Expended when work is performed
e.   All of the above
18. Air mass or single cell thunderstorms are short-lived because

a.   The are vertically-oriented
b.   They produce little rain
c.   The gust front quickly outruns the updraft
d.   They form in stable environments
e.   All of the above

19. Relative humidity decreases when the air temperature increases if


a.   The amount of moisture in the air remains constant
b.   The amount of moisture in the air decreases
c.   The dew point drops
d.   The absolute humidity increases
e.   Both a and c

20. Temperature is
a.   The average speed of atoms and molecules
b.   A byproduct of sensible heating
c.   A byproduct of latent heating
d.   Always transferred from hot objects to cold ones
e.   Both a and b

21. Which requires more energy to complete the stated phase transition?

a.   Ice to liquid water
b.   Ice to water vapor
c.   Liquid water to water vapor
d.   Liquid water to ice
e.   Liquid water to slush
22. The albedo of Earth is, on average

a.   40%
b.   50%
c.   60%
d.   100%
e.    None of the above

23. Relative humidity (RH) is NOT a good measure of the amount of moisture in the air because

a.   RH depends upon the vapor pressure
b.   RH depends upon the temperature
c.   Cold air can hold far less moisture than warm air at the same pressure
d.   The RH is a function of the dew point
e.   None of the above
24. Which of the following is/are independent of air parcel volume

a.   Relative humidity
b.   Mixing ratio
c.   Dew point
d.   Specific humidity
e.   Both b and d

25. Relative humidity is

a.   The saturation vapor pressure divided by the vapor pressure
b.   The saturation mixing ratio divided by the mixing ratio
c.   The vapor pressure divided by the dew point
d.   The temperature divided by the dew point
e.   None of the above

Part II: True/False (1 point each) - Indicate the correct answer on the computer form using
a #2 pencil.

26. Cement has a higher heat capacity than water
a. True
b. False

27. The saturation vapor pressure depends primarily upon temperature.

a. True
b. False

28. Thunderstorms are unable to penetrate very far into the stratosphere because, above the
tropopause, the temperature of the atmosphere increases with height.

a. True
b. False
29. Warm air has the capacity to hold less water vapor than cold air at the same pressure.

a. True
b. False

30. In an unstable atmosphere, air parcels displaced vertically will oscillate around their point of

a. True
b. False

Part III: Definitions (3 points each) - Provide a short definition for each term in the space

1. Dry adiabatic lapse rate
The rate at which the temperature of an unsaturated air parcel changes as it rises or descends.
The value is approximately 10 degrees Celsisus per kilometer (specification of this value is not
required in the answer).

2. Convective available potential energy

The amount of energy contained in the environment that is available to a parcel rising vertically,
or the summation of the temperature difference between a parcel and the environment over the
depth for which the parcel is warmer than the environment.

3. Latent heating

The process by which energy is released when a substance changes phase, say from a gas to a
liquid in the case of water vapor condensing to liquid water. (Latent heat is the amount of
energy required for a change of phase.)

4. Conditional instability

The atmosphere is said to be conditionally unstable when a parcel of air rising dry adiabatically
is stable, and when the same parcel of air, if rising moist adiabatically, is unstable.
5. Saturation

The physical state at which a parcel of air can hold no more water vapor without condensing.
Under saturated conditions, the relative humidity is equal to 100%,

Part IV: Short Answer (10 points each) - Provide a concise answer to each of the questions
listed below.

1. Suppose that an air parcel, located at the ground and having a temperature of 45 degrees C,
begins rising dry adiabatically in an atmosphere in which the temperature decreases at a rate of 5
degrees C per kilometer. If the atmospheric temperature at the ground is 35 degrees C, above
what altitude will the rising air parcel become colder than the environment?
The best way to handle a question like this is to draw a picture, as shown below, which clearly
illustrates the decrease of temperature with height for both the parcel and environment. The
parcel and environment both have temperatures of 25 degrees Celsius at an altitude of 2000 m,
so above this height, the siring parcel becomes colder than the environment, as can be confirmed
by comparing the temperatures at 3000 m.

2. Illustrate and briefly describe the three stages of an air mass thunderstorm, noting in particular
the reason(s) for their relatively short lifetime.

Cumulus Stage: The cumulus stage is the initial stage in the development of thunderstorm
formation. This stage is dominated by updrafts, which are caused by saturated air that
"streams" upward throughout the cloud. As each new surge of warm air rises higher than the
last, the height of the cloud increases. It only takes about 15 minutes for the tops of the cumulus
clouds to reach altitudes of 8,000 to 10,000 meters. Since the updrafts are strong enough to
suspend water droplets and ice crystals in the air, precipitation does not occur during the
cumulus stage. When the accumulation of the precipitation in the cloud becomes too heavy for
the cloud to support, it begins to fall causing a drag on the air. This initiates a downdraft.

Mature Stage: The mature stage begins once the precipitation hits the earth's surface. This
stage is the most active period of a thunderstorm. Updrafts exist side by side with down drafts
during this stage, which causes the cloud to continue to grow. Before the actual precipitation
reaches the ground, the cool down draft leaves the base of the cloud and spreads horizontally at
the surface to create the gust front. The updrafts spread horizontally as the cloud grows to the
top of the unstable region (usually located at the base of the warmer stratosphere) and the "anvil
top" forms. This anvil top is made up of mostly ice-laden cirrus clouds which are eventually
spread downwind by rapid winds aloft. Toward the end of the mature stage, the cell reaches it's
maximum intensity. During this time rain is the heaviest, hail and strong surface winds may
develop, as well as weak tornadoes.

Dissipating Stage: Once the downdrafts begin to dominate throughout the thunderstorm cloud
the dissipating stage is formed. Thunderstorm activity ceases as the influx of colder air and the
cooling effect of falling precipitation occur. During this stage, subsiding air replaces the updraft
throughout the cloud. This cuts off the supply of moisture provided by the updraft, causing the
cloud to eventually evaporate.

Major Point: Single-cell storms form in environments where the winds are relatively weak
throughout the depth of the atmosphere, or change little in altitude with regard to speed and/or
direction. Because of this structure, the precipitation that forms during the mature stage falls
back on top of the updraft, thereby destroying it. It is for this reason that airmass storms have a
built-in self-destruct mechanism and thus relatively short lifetimes (approximately 45 minutes).

3. Briefly describe (and use illustrations if you like) why microbursts pose a threat to aviation.
Microbursts pose a threat to aviation because they induce rapid changes in wind speed and
direction close to the ground, usually in the landing phase of flight when the aircraft is flying
slow with wheels down and flaps out. Consequently, it is difficult for an aircraft to recover from
an intense microburst because, as shown in the figure below, the lift over the wings (which is
proportional to the square of the wind speed) is greatly diminished as the plane crosses the axis
of the downdraft into the tail wind.

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