WELCOME BACK: Procedure continued from STUDY GUIDE.
Rising and sinking air motions occurring in the atmosphere may be produced by several situations. Rising motion can be caused by approaching fronts, convergence of winds blowing into a surface low pressure center which forces air aloft, localized daytime solar heating, or orographic flow over rising terrain. Conversely, sinking motion can occur with near-surface divergence of air in Highs or by air flow over descending terrain. The Monday 15 October Activity 6A dealt with the atmosphere over Dulles International Airport as a strong cold front approached. Rising motions caused by the approaching front produced [(clouds) (clear conditions)].
The Stüve diagram of the sounding from Dulles Airport delivered as Image 2 of the Monday Activity 6A shows [(saturated) (unsaturated)] conditions through most of the troposphere over Dulles. The temperature patterns in the atmospheric layer from about 910 mb to 810 mb and again in the layer from 790 mb to 570 mb closely parallel [(saturated) (unsaturated)] adiabatic lines.
Image 1 of this Activity 6B is the Stüve diagram of the sounding from Dulles International Airport (IAD) for 1200Z 17 OCT 2001. At this time, Dulles was to the east side of the high pressure center of the cool air mass that followed the passage of another cold front across the eastern US. Details of the weather patterns for this time including this frontal passage can be found in the Wednesday, 17 October 2001, Daily Weather Summary. The winds at various atmospheric levels inferred from the radiosonde's drift during its ascent are plotted along the right side of the graph at altitudes represented by tic marks on the vertical axis. The direction for each wind arrow follows the surface station model where north is to the top of the page. The surface wind is from the [(west-northwest) (southeast)] at 10 knots. This direction [(is) (is not)] consistent with that expected following the passage of a cold front.
From the surface (1005 mb) to 990 mb, the temperature [(is decreasing) (remains constant) (is increasing)]. This is a temperature inversion. Surface temperatures cooled due to overnight radiation under clear skies. Other inversions exist from about 850 mb to 830 mb and 710 to 700 with an isothermal layer from 700 to about 680 mb.
At the surface, air at Dulles [(was) (was not)] saturated as evidenced by the separation of the temperature and dewpoint values on the Stüve diagram. Therefore, air rising from the surface at Dulles would initially be a(n) [(unsaturated or "dry") (saturated)] air parcel. This rising air would therefore cool at the [(dry) (saturated)] adiabatic rate.
In the layer just above the surface (990 mb) up to about 860 mb, the temperature profile is approximately parallel to the solid adiabatic lapse rate lines on the Stüve. Assuming rising air, this path indicates the ascending air was cooling at the [(dry) (saturated)] adiabatic lapse rate.
DataStreme Activity 6A (Clouds, Temperature, and Air Pressure) investigated the relationships between air temperature and air pressure changes. The Stüve diagrams used in the 6A and 6B activities graphically display these relationships with sets of lines drawn to represent temperature changes which occur when saturated or unsaturated air undergoes vertical motion. The radiosonde soundings for Dulles which you have just examined show the contrasts between atmospheric conditions near fronts and within air masses. Ascending and descending air actually behaves according to these relationships!
You might have your students print out the text data of rawinsonde observations and plot them on a blank Stüve diagram (available on the DataStreme homepage) when Highs, Lows, and fronts pass nearby. Then compare your cloud and sky conditions with the temperature profiles you have plotted.
After completing this week's applications, fax the following pages to your LIT mentor by Monday, 22 Oct. 2001, or as coordinated with your mentor:
You may wish to trace the temperature processes of Activity 6B on the Stüve (STUDY GUIDE, Part B: Applications, page 6B-2) and fax it to your mentor for review along with those pages above if you have questions.
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