DataStreme Activity 6A:

CLOUDS, TEMPERATURE, AND AIR PRESSURE

Do Now:

  1. Print this file.
  2. Print the Monday Image 1 and Image 2 Files.
  3. Print (when available) the Tuesday, 16 October 2001, Daily Summary File.

To Do Activity:

  1. Read Chapter 6 in STUDY GUIDE, Part A: Narrative.
  2. Go to STUDY GUIDE, Part B: Applications. Start Activity 6A.
  3. Return here (Monday Activity A File) when told to do so.

Go To STUDY GUIDE - Activity 6A Now


WELCOME BACK: Procedure continued from STUDY GUIDE.

  1. At any given temperature, there is a maximum concentration of water vapor that may be in a volume. This condition, called saturation, occurs when the temperature and dewpoint (temperature to which the air would have to be cooled to have a relative humidity of 100%) are equal. Air always contains some amount of water vapor, usually less than the maximum possible for its temperature. Cloud formation, whether in the atmosphere, or at Earth's surface (fog), requires this saturation condition so that water vapor can change to the liquid (or solid) state. Thus, the atmosphere must undergo some process whereby saturation is achieved.

    The first part of this activity had you investigating how atmospheric processes can produce saturation by changing air pressure. Decreasing pressure on an air volume leads to expansional [(cooling) (warming)]. Therefore an air parcel containing water vapor and rising through the atmosphere would expand and cool to the dewpoint. Your release of pressure on the bottle resulted in cooling and produced saturation - hence, your cloud! Increasing the pressure, as a descending air parcel would experience, warmed the air by compression to temperatures above the dewpoint, and your cloud evaporated.

  2. On Sunday, 14 October 2001, a strong cold front was crossing the eastern US. This front brought severe weather to the southern Plains earlier in the weekend and heavy rains along its length. Details of the front's passage across the country are given in the Monday, 15 October 2001 Daily Weather Summary.

    Image 1 is the surface map for 00Z 15 OCT 2001 (8 PM EDT Sunday evening). This map shows the position of the cold front at 00Z to be just west of Dulles International Airport near Washington, DC. The temperature at Dulles was 66 °F with a dewpoint of _______ °F. The sky conditions were [(partly cloudy) (overcast)] and winds were from the south-southwest at about [(5) (10)] knots. This wind direction [(is) (is not)] consistent with the front not yet passing the station.

  3. The condition where the temperature and dewpoint at Dulles Airport were equal indicates the near-surface air [(was) (was not)] saturated. Rain was also reported at Dulles at that hour. The sky coverage and rain indicated there [(was) (was not)] saturated air above ground level at the time.

  4. The cold front approaching Dulles was the mechanism for lifting the air along the eastern US. With lifting comes [(compressional warming) (expansional cooling)] and cloud [(formation) (dissipation)].

  5. Image 2 is the Stüve diagram from Dulles International Airport (IAD) rawinsonde observation at the same time as the surface map. The plotted curve to the right (higher values) on a Stüve is the vertical temperature profile; the curve to the left (lower values) is the vertical dewpoint profile. (Temperatures and dewpoints are read by using the same scale in degrees Celsius appearing across the base of the diagram.) The plotted dewpoint is the temperature to which the air would have to be cooled at constant pressure to become saturated with water vapor.

    When the two curves are identical (one on top of the other), the air temperature and the dewpoint have the same numerical value. Therefore, the air is [(saturated) (unsaturated)]. The temperature and dewpoint curves at the surface (about 1000 mb) are equal. The curves are also equal from about 910 mb up to 500 mb. Therefore clouds probably [(did) (did not)] exist through that section of the troposphere over Dulles at the time of the sounding. (This is consistent with the sky coverage reported on the surface map at that time!)

  6. The relative humidity of the air at the surface at Dulles was [(less than) (equal to) (greater than)] 100%. The relative humidity of the air over Dulles from about 910 mb to 500 mb was [(less than) (equal to) (greater than)] 100%.

  7. We can assume from our experience with the first portion of this activity that [(upward) (downward)] vertical motions were occurring in the atmosphere over Dulles, produced by the approaching cold front. These ongoing vertical motions produced 1.10 inches of rain in the six-hour period around 00Z as the front passed through the Washington area.


When a period of fair weather or storminess affects your location, have your students call up Stüve diagrams for the available upper-air station nearest you to determine what is occurring above the surface in your region. You can also look for indicators of high atmospheric humidity at the surface such as "fogging" of windows or condensation forming on cold drink cans. Clear, calm overnight hours may also see the formation of frost as we encounter cooler temperatures. Compare these visual indicators of atmospheric humidity with DataStreme surface data which includes dewpoints reported for your nearest station.


Hold this activity until you have completed all applications for this week. Instructions for faxing your LIT mentor will appear at the end of this week's Activity B.


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