Quiz 7 Review: Answers
     
  • Thunderstorms-

    • #1. See Figure 10.1 and the handout on the three stages of an ideal air mass thunderstorm. Be sure to include the major characteristics of each stage (i.e. updrafts, downdrafts, temperature of various regions, anvil) and indicate where precipitation might be occurring in those stages that have precipitation.
    #2. The typical life-time of an air mass thunderstorm is about 1 hour.
    #3. Downdrafts are started when the precipitation particles are too large to be supported by the updraft.  So they begin to fall. As they fall air is dragged downward with the particle due to friction.  As the particle continues to fall it melts/evaporates.  In doing so energy is taken from the surrounding air, thus cooling the air. This cool air is heavier than the air around it so it will sink a little faster. This melting/evaporation intensifies and maintains the downdraft. Air from outside the cloud is also "entrained" into the cloud and since this air is usually unsaturated.  This too will cause the cloud droplets and precipitation particles to evaporate and cool the air.
    #4-5. One of the requirements for thunderstorm formation is unstable air. This means we want cool air over warm air.  Over land we will get this most likely when the surface is the warmest, the afternoon.  Now over the ocean, since the specific heat of water is so high the temperature of the ocean water does not change much over the course of the day. So, to get the picture of cool air over warm air, we now want the time at which the air above will be the coolest since the air at the bottom will not change it's temperature much.  The air above will be the coolest during the night or earliest part of the morning.  In both cases we still want unstable air, cool over warm, they are achieved by different means.
    #6. Figure 10.18 of the text shows that Florida has the highest occurrence of thunderstorms per year.  Most, but not all, of these thunderstorms occur during the summer.  Here during the summer the air over the land is warmer than air over the ocean.  So as the warm moist air over the land rises, a region of slightly lower pressure is created over the land compared to the air over the ocean.  So, air flows from the ocean to the land due to the difference in pressure.  Since Florida is essentially surrounded by ocean it has air coming from both sides and converging over the land.  This convergence of air at the surface over the land helps to lift air above the level of free convection where the air will rise upon its own.  This sort of picture happens along the rest of the Gulf Coast States, but to a lesser extent.
         Now the same sort of land - ocean pressure difference occurs during the summer along the West Coast, but here the air has less moisture because the ocean water is cooler than the Gulf of Mexico and the South Western Atlantic Ocean.  As a result we don't see nearly as many thunderstorms along the West Coast.
    #7. Downdrafts can "spawn" new thunderstorms if the downdraft is not collocated with the updraft that is supplying the warm moist air for condensation. This can happen when the downdraft is "tilted" as in Figures 10.6 and 10.7 of the text.  Technically, these two figures show how the downdraft can allow a storm to live more than the 1 hour of the ideal air mass thunderstorm.  Two downdrafts, each from a different storm, that meet produce converging air at the surface and can provide the lifting force for surface air to reach the level of free convection.  This would produce a new thunderstorm.
    #8. Precipitation that falls from a storm cloud, but does not reach the ground is called virga.
    #9. A microburst is an intense downdraft.  Virga can contribute to a microburst by the precipitation evaporating and cooling the air and causing it to become much more dense.  Thus increasing the rate at which the cool air of the microburst "falls" to air.  Fujita once described an microburst as a cold air balloon.
    #10. A gust front will:
        -Bring a shift in the wind direction
        -Often cause the wind to increase in speed
        -Cause the pressure to increase slightly
        -Cause the temperature to decrease
    #11. The expression "vertical sheer" refers to the ambient wind changing either speed with altitude and/or direction with altitude.  An environment with this sort of conditions will cause the updrafts and downdrafts of a thunderstorm to be "tilted" and thus not interfere with each other.  This is illustrated in Figures 10.6 and 10.7 of the text.
    #12. Mesoscale Convective Systems include; super cells, squall lines, multi-cell thunderstorms, tropical cyclones, and Mesoscale Convective Complexes.  A super cell differs from the other members this group in that it is a single cell thunderstorm.  In addition, this single cell has some rotation, but this is not a tornado.
     
  • Lightning-

    • #1. The most common form of lightning discharge takes place within a a cloud.
    #2. Lightning is more frequent over the land masses of the globe.
    #3. See page 258 of the text and Figure 10.21.
    #4 Lighting occurs in clouds that extend above the freezing level;
    Charge separation is restricted to precipitating clouds;
    The charge regions within the cloud are generally positive in the upper part of
    the cloud and negative in the middle to lower part of the cloud.
    #5. The most widely accepted theory for the charging of thunderstorms is the Thermoelectric effect.  See page 258 of the text for the details.
    #6 For every mile there is a 5 second difference between seeing the flash and hearing the thunder.  So, 25 seconds/(5 seconds/mile) = 25/5 miles = 5 miles.
    #7. In the U.S. the highest frequency of lightning on a yearly basis is in Florida.
    #8 Thunder is caused by the rapid heating of the air surrounding the lightning channel.  This air then expands rapidly and we hear the resultant shock wave or sound wave.
    #9 The light from a lightning flash is caused by the recombination of air ionized by the discharge and free electrons.
  • Tornadoes-
    1. False: Tornadoes only occur in the United States.
    2. True: A tornado can have more than one vortex.
    3. False: Tornadoes only occur during the summer and spring.
    4. False: All tornadoes are produced by super cell thunderstorms.
    5. In the U.S., the highest number of tornadoes reported per year per square mile is in Oklahoma with an average of 7.9 per 10,000 square miles. See Figure 10.27 of the text.
    6. Know typical values for the following characteristics of a tornado:

      7. - Diameter: 300-2000 ft.
      - Preferred direction of movement in the U.S.: from southwest to northeast
      - Speed of propagation: 20-45 mph.
      - Wind Speeds: 40-100 mph.
      - Severity according to the Fujita Scale: F0-F1