Notice: Figure 4.4, p. 63 of the Part A: Narrative (text) should have the shortwave reflected by clouds as 17 units. (A printed smudge mistakenly looks like a decimal.)
WELCOME BACK: Procedure continued from STUDY GUIDE.
As solar radiation decreases in the middle and upper latitudes of the Northern Hemisphere at this time of year, the cooler surfaces create cooler air masses. These air masses and their circulation patterns then migrate to lower latitudes bringing cooler air to a region. The accompanying cold air advection often causes dramatic temperature drops for a location over several hours or a day. In similar fashion, warm air advection may cause dramatic warm-ups.
Image 1 is the map of reported surface weather conditions for 12Z 24 SEP 2001 (7 AM CDT). This map shows the most recent example of a dramatic cool-down across the eastern half of the coterminous US. At map time, a low-pressure center was shown to the east of Lake Huron with a [(cold) (warm)] front stretching southwestward to another Low in southern Mississippi and on to the Rio Grande Valley between Texas and Mexico. From the Great Lakes Low, a [(cold) (warm)] front curved northeastward to eastern Canada.
The wind directions at the stations surrounding the Great Lakes Low show the circulation to be [(counterclockwise and inward) (clockwise and outward)]. Located north of Minnesota, along the Canadian border, is a high-pressure center. The wind directions at the stations in the region surrounding the High show the circulation to be [(counterclockwise and inward) (clockwise and outward)].
Temperature conditions at stations are shown in the surface station model described in Activity 2A. The temperature value plotted to the upper left of the station circle at Green Bay, Wisconsin, was [(62) (50) (40)] °F. The wind direction at Green Bay was from the [(north-northwest) (south)]. The temperature value plotted at Detroit, Michigan, was [(62) (50) (40)] °F. The wind direction at Detroit was from the [(northwest) (south)].
Thus, the wind flow directions across the region from Wisconsin to western Ohio as exemplified by these stations was from regions of [(higher) (lower)] temperatures. [(Cold) (Warm)] air advection was occurring at these locations as the wind was blowing from colder to warmer regions.
Now observe the wind directions and temperatures across the region from western New York to Massachusetts and Long Island. Wind directions were from the [(north or northwest) (south or southeast)]. The temperatures varied from 70 °F at New York City to _____ °F at Ottawa, the capital of Canada, just northeast of Lake Ontario. The winds were flowing from regions of [(higher) (lower)] temperatures. [(Cold) (Warm)] air advection was occurring at these locations as the wind was blowing from warmer to colder regions.
The region of cold air advection was generally located [(to the west of or "behind" the cold front) (to the south of or "behind" the warm front)]. This cold air advection location was also generally to the [(eastern) (western)] side of the High.
The region of warm air advection was generally located [(to the west of or "behind" the cold front) (to the south of or "behind" the warm front)].
Image 2 is the surface weather map (Isotherms, Fronts, & Data) for 12Z 01 OCT 2001, the conditions at the beginning of this week. This surface map depiction shows station model information, the locations of surface weather systems within a few hours of the surface data, and the analysis of the temperature pattern by way of computer-drawn isotherms at 10 F°-degree intervals.
Temperatures on the Image 2 map from northern Florida across to Texas are generally in the [(upper 40's and lower 50's) (lower 70's and upper 60's)]. Meanwhile temperatures from New England across to the Dakotas are generally in the [(middle 40's and lower 50's) (lower 70's and upper 60's)]. Thus, there are no great temperature differences over this expanse of country.
A temperature gradient is the change of temperature over horizontal distance. On the Image 1 map of 24 SEP there were relatively great temperature changes over short distances behind the cold and warm fronts. On the Image 2 map of 01 OCT, there were only small changes of temperature over great distances in the eastern two-thirds of the US as shown by the widely spaced isotherms. (In the West, relatively close isotherms reflect the effects of mountainous topography.)
Warm or cold air advection depends on the strength of the temperature gradient, the speed of the winds, and the angle of the wind across the isotherms. Light winds, as may occur overnight, or near the center of Highs, produce little temperature advection. As weather systems cross your region in the coming weeks and months, have your students call up DataStreme products to identify patterns of cold or warm air advection associated with these passing systems. The DataStreme homepage provides a map of station models and computer-analyzed isotherms ("Isotherms, Fronts, & Data") that aids in finding warm or cold air advection.
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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©Copyright, 2001, American Meteorological Society