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, 17 October 2000, 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 (related to the maximum possible water vapor) 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 has 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, warms the air by compression to temperatures above the dewpoint, and your cloud evaporated.

  2. Image 1 is the Stüve diagram from the Detroit, MI (DTX) rawinsonde observation for 0000Z 16 OCT 2000. 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 that pressure to become saturated with water vapor. Consequently, the dewpoint can never be greater than the temperature at the same level!

    The approximate values at the surface (lowest level, 980 mb) are - temperature: [(11) (14) (18)] °C, dewpoint: [(11) (14) (18)] °C. The temperature and dewpoint at the surface [(are) (are not)] equal implying the air at the surface [(is) (is not)] saturated.

  3. When the two curves are close together the difference between the air temperature and the dewpoint is [(large) (small)]. The temperature and dewpoint curves from about 950 mb to about 520 mb are are equal or within about one degree Celsius of each other. Therefore conditions of [(high) (low)] relative humidity existed through that section of the troposphere over Detroit at the time of the sounding.

    Meteorologists know from experience that clouds probably exist when the temperature and dewpoint reported on a Stüve for a particular altitude are equal or within a few degrees of each other (less than five). When this occurs, it indicates that the air is either at or near saturation and therefore, the relative humidity is [(high) (low)]. The Image 1 DTX Stüve indicates that there probably [(were) (were not)] clouds from about 950 to 520 mb above the Detroit area. (The complete equality often is not attained due to humidity sensor response.)

  4. As mentioned in the Monday, 16 October 2000, Daily Weather Summary and seen in the thumbnail satellite and fronts image of 0015Z 16 OCT 2000 with the Summary, a stationary front was across Detroit in the region of the lower Great Lakes. Airflow from the south over this frontal surface provided lift to the air. This rising air [(warmed) (cooled)] on [(expanding) (compressing)], like the air in your bottle when pressure was released. This air first became saturated at about [(950) (500)] mb.

  5. Above about 500 mb, the temperature and dewpoint curves are [(more) (less)] than a few degrees apart. This would suggest there [(were) (were not)] saturated conditions and clouds above that level. In fact, the "Y"-shape of the curves near 500 mb implies that air above 500 mb was sinking. Sinking air would be compressed and would [(warm) (cool)]. When you compressed your bottle, the cloud that had formed [(evaporated) (became denser)]. (Additional evidence for this sinking motion is the isothermal layer from about 500 mb to 460 mb. The atmosphere usually has a consistently cooling temperature profile with altitude.)

  6. Yet another way saturation is achieved can be seen in Image 2, the meteogram for Mobile, Alabama (MOB), from 1300Z 15 OCT to 1200Z 16 OCT 2000. This time series of surface weather conditions at Mobile shows the temperatures and dewpoints during this 24-hour period on the upper panel of the meteogram and sky coverage and winds in the second panel. At 20Z (3 PM CDT) 15 OCT, the temperature was about [(78) (83)] °F while the dewpoint was about [(48) (55)] °F . This difference between the temperature and dewpoint meant that, the relative humidity was fairly [(high) (low)]. This afternoon warming occurred under skies that were [(clear) (overcast)] and winds that were [(light) (strong)].

  7. From 20Z (3 PM CDT) 15 OCT to 06Z (1 AM) 16 OCT, the temperature steadily [(fell) (rose)]. From 23Z to 03Z while the temperature was falling, the dewpoint [(remained steady) (fell)]. This changing difference between the temperature and dewpoint meant that, over this latter period, the relative humidity was [(falling) (rising)].

  8. By 05Z 16 OCT , the temperature and dewpoint were within a degree or two of each other. This difference between the temperature and dewpoint meant that the relative humidity was [(low) (at or near 100%)]. In the third panel of the meteogram, VISIBILITY and WEATHER, the weather symbol for ______ (broken line over solid line) was reported at 05Z. This is consistent with the saturation conditions. (Weather symbols may be found in the User's Guide, Extras, Map Symbols, linked from the Homepage.) During the 06Z - 12Z period, the horizontal visibility decreased from the earlier 10 miles to [(0) (4) (8)] miles.

  9. Until the formation of dense fog at 06Z, the sky cover remained [(clear) (partly cloudy) (overcast)] and winds were [(less than 5) (more than 15)] knots.

    During the early morning hours, the air at Mobile was brought to saturation by radiational cooling. The Gulf Coast had remained under the influence of the high pressure that had been the major weather maker for the Eastern US all of the preceding week. The lower panel, PRESS, showed that there was little pressure change at this time. The lack of significant pressure change means that expansional cooling was not involved. The relatively steady dewpoint means that no water vapor was added to the air. Radiational cooling of air to its dewpoint, at essentially constant pressure, demonstrates another way whereby saturation occurs.


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. 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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