A typical scene!
This is an additional statement on the special projects for the class. Successful completion of one of the options below will excuse your 2nd lowest quiz grade. I'd like these handed in for options 1,2,3 at the time of the next and last quiz, Thurs 18 April. (Option 4 is due Thurs 25 April.)
Option 1: Maintain a record of temperature, dew point, wind, and weather for College Station (CLL) or any other station of your choice, for the period from 1-15 April. (I'd accept a later period for those who couldn't get started before 8 April; say 5-18 April.) I'm not insisting on any specific format, but don't do so much that you overwhelm me with paper (or lengthy emails) and don't do so little that your record is full of holes that miss important changes or events. The main purpose of this record is for you to evaluate, each day, what air mass is affecting your station, and if it has been modified, how so. Also, identify any frontal passages, with reasons for your conclusions. If there is significant rainfall or other interesting weather event, relate that event to the air masses and fronts.
Option 2: Each day from 1-15 April, scan the weather maps for the United States (the contiguous 48 states thereof) and identify those locations or regions which have a risk of severe weather during the next 24 hours. State the basis for your conclusions. By "severe weather" we mean thunderstorms with winds in excess of 50 knots, hail larger than 3/4 inch in diameter, or tornadoes. You might also comment on the possibility for excessive rainfall, flash floods, blizzards, or any really unusual events. Factors you should be considering include the following:
(a) Severe thunderstorms generally require both very unstable air and significant change of wind direction and speed with height. Therefore, anywhere in a maritime tropical air mass where there is (typically) a strong (approx) south or southeast wind at the surface or 850 mb, with strong southwest-to west wind at 700 mb or 500 mb would be a possible area.
(b) The conditions described under (a) above indicate that thunderstorms will severe IF THUNDERSTORMS OCCUR AT ALL. It is quite common for such conditions to be present over a large area, but without sufficient LIFTING of the air mass to release the instability. Therefore the other half of the problem is to look for places where the warm, moist, unstable air will be lifted. As we have already discussed, fronts and upper-level troughs can often provide such lifting. There are other factors to consider which will be discussed as we cover Chapter 15.
Option 3. Any time between 1-17 April, identify an actual outbreak of severe weather somewhere in the contiguous 48 states. Ideally for this exercise it should be a significant one, with severe thunderstorms having affected quite a large area, or having done considerable damage. Using mainly the "weather" program on "glory", evaluate how well the National Weather Service did on their watches and warnings. (Ask me for some short cuts I can give you if you are interested in this option.)
Option 4. (due April 25). Read several references that deal with global climate change or "global warming", and write a brief, concise summary report which outlines some of the major issues. The report should be no longer than 3 pages, with at least 2 references. You should prepare one or two transparencies which you can use to outline your conclusions in a 1-2 minute oral summary to the class on April 25. If you are interested in this option, please see me in my office; I can get you started with several books and/or reports.
Regarding the first option for the 1-15 April Weather Project, "Keeping track" of air masses at a particular station (for many of you, probably College Station, CLL).
What I hope you learn from this exercise is just how much of the air mass and weather changes which occur are easily related to "classical" air masses and fronts, and how many are not so easily described. Let me try an example or two.
Suppose the weather for a few days is as follows:
Morning T/Td Afternoon T/Td Wind/Clouds/Weather Remarks
(Temp/Dew Point)
April 31: 25/23 45/22 Calm/Clear Frost in am
April 32: 41/40 57/46 Light SE wind, low clouds am
Strong SE wind, low clouds pm
April 33: 63/63 72/65 SE wind, light rain, drizzle am
SE wind, scattered cumulus pm
Strong thunderstorm evening
April 34: 43/32 33/23 North wind blowing students
around O&M building all day
I hope that you would go beyond the obvious, that is to say, that we were under
the influence of a cP air mass on April 31 and 34th, and an mT air mass on
the 33rd. Please explore how the transitions occurred. For example, the
transition from cold/dry to warm/moist took 2 full days and seemed to
happen gradually in this example, so there was probably not a sudden
change, as if a sharp warm front had passed. You should reinforce this
conclusion by looking
at the hourly data (which would show whether the change was or was not
gradual), or by commenting on the weather maps which might have shown a
large high pressure zone with no sharp air mass boundaries. You might also
comment on the trajectory of the air coming from the SE....presumably when
it was cool on the 32nd it had a very short trajectory over the Gulf, but
by the 33rd it must have spent some time over warm water.
The thunderstorm the evening of the 33rd may or may not have marked the
passage of the cold front and strong norther (and shocking air mass change
for April).
Examine the weather map, and the hourly data, to find out whether the
temperature started to drop sharply at the same time as the thunderstorm,
or whether it was a pre-frontal squall line. IN the latter case, you would
observe a moderate drop in temperature from rain and downdrafts during the
storm (say from 73 to 60), near-constant temperatures for several hours,
and THEN a second wind shift, pressure rise, and sudden temperature drop as
the real cold front arrives.
Hope that helps. Comments and questions are welcome!
Reminder of some important principles, which are valuable not just for these chapters,
but for entire course:
Sinking Air is associated with clear or clearing skies, warming and drying. WHY? Because as air sinks, recall that it warms by compression at the dry adiabatic lapse rate of 10 degrees C per kilometer of sinking. Its saturation vapor pressure (or saturation mixing ratio) (recall Figure 5.11 or Fig 7.19) while the actual moisture content (whether measured by vapor pressure, mixing ratio, or dew point) remains approximately constant. Therefore the relative humidity decreases, and any clouds evaporate.
Rising Air is associated with clouds and precipitation. Reversing the reasoning just above for sinking air, rising air cools by expansion at the dry adiabatic rate until saturation is reached, then at the moist adiabatic rate. The depth of the clouds will be related to the depth of the rising saturated air mass, and the precipitation rate will be related to that, and especially to the magnitude of the upward vertical velocity.
In-Up-Out Remember that for air to rise there must be low-level convergence (in) and upper-level divergence (out). In-Down-Out For air to sink there must be upper-level convergence (in) and lower-level divergence (out).
Convergence usually means that cyclonic spinup must occur That is the main reason that low level convergence, clouds, and precipitation are so often associated with air entering a cyclonic circulation at low levels.
What kind of weather results from rising moist (Maritime Tropical) air? That depends! Review Chapter 7 on stability. If the air is stable, overcast with steady rain. If the air is conditionally or convectively unstable, .....KABOOM! (See Chapter 15.)
Fronts are important for many reasons. They mark the transition zones from one air mass to another, and they are often the locations for moist air to rise. But they are not the only way to get air to rise. Regardless of the cause (dry lines, fronts, upper-level troughs, sea breezes, air rising over mountains, intertropical convergence zone) if moist air is rising, look for interesting weather.