|
AIR PRESSURE AND WIND |
Welcome to the preview week of AMS Weather Studies. We hope the use of current weather information will become a pleasant and anticipated daily experience. We encourage your exploration of the online weather products from the website.
The Image 1 "U.S. - Data" map acquired from the AMS Weather Studies website is the depiction of weather conditions at stations across the contiguous U.S. at 12Z 25 AUG 2008. [12Z is four hours ahead of Eastern Daylight Time (EDT), so the map depicts conditions at 8 AM EDT (7 AM CDT, 6 AM MDT and 5 AM PDT).] At map time weather conditions were mainly fair across most of the U.S. Some clouds and a few scattered showers were located in the extreme Northwest States associated with a frontal system coming onshore from the Pacific Ocean. The major story, however, was the continued presence of Tropical Depression Fay along the Gulf of Mexico coastal states. Fay formed ten days prior to maptime over the Dominican Republic and Haiti, became a tropical storm and proceeded to set a record, making four subsequent landfalls in various parts of Florida. Fay's persistence in its slow meanderings along the Gulf Coast has resulted in prodigious rainfalls and serious flooding. Details of this weather system are given in the Monday, 25 August 2008, Daily Weather Summary file.
Weather data at individual locations are plotted in a coded format called the "station model." The wind directions at reporting stations on the map are shown by the line (which can be thought of as an arrow shaft) which depicts the air flow into circles representing station locations. Wind at a station is named by the direction from which the air flows, i.e., air arriving at the station from the north is a north wind. The wind direction at Salt Lake City, Utah, at map time was from the [(southeast) (northeast)].
(All reporting surface weather stations can be identified from the "Available Surface Stations" link on the website and identities given in the "User's Guide." Also a map of National Weather Service offices can be found at: http://www.wrh.noaa.gov/wrh/forecastoffice_tab.php)
Given the direction the wind at Salt Lake City was from, it would be reported as a [(southeast) (northeast)] wind
The wind speed is given by a combination of long (10 knots) and short (5 knots) "feathers" on the direction shaft. [The station model will be explained in Investigation 2A. Further details for deciphering station data can be found in your User's Guide (linked from the AMS Weather Studies website).] At map time, Salt Lake City had a 15-knot wind (one long and one short feather). [A double circle without a direction shaft signifies calm conditions, such as Casper, WY and Des Moines, IA, and a shaft without feathers denotes 1-2 knots. One knot (nautical mile per hour) is about 1.2 land (statute) miles per hour.]
Mark a bold "L" about 1 cm in height in east-central Mississippi (over the "029" of the Jackson station model). Use a red marker if possible as Lows are traditionally shown on weather maps in red. Compare the hand-twist model of a Low to the wind directions in the several station area about this low-pressure center. Wind directions at stations across this area of the central Gulf Coast states show that, as seen from above, the air spiraled generally [(clockwise) (counterclockwise)] around this low-pressure center, denoted by your L.
The wind directions around the low-pressure center also indicated that the air generally spiraled [(inward toward) (outward from)] the low-pressure center.
This wind flow pattern about the Low is [(consistent with) (contrary to)] the hand-twist model of a Low.
The local coverage of the sky by clouds at a station is denoted by the shading within the circle representing the station. A dark circle means overcast conditions, i.e. completely cloudy sky. An open circle means clear skies. Partial shading represents the fraction of sky covered by clouds. The skies in the immediate area about the Low were generally [(clear) (cloudy)].
The hand-twist model of a Low includes vertical motions with air rising. Based on the Low shown on this map, areas of rising air are likely to be locations of [(clear) (cloudy)] skies.
This pattern of cloud cover [(was) (was not)] consistent with low-pressure systems being "stormy" implying extensive cloudiness and possibly precipitation. (Note the four dots denoting heavy rain to the left of the Mobile, AL station circle at maptime. This Low marked the position of Tropical Depression Fay.)
Mark a bold "H" about 1 cm in height just east of the "222" on the north shore of Lake Superior. Use a blue marker if possible as Highs are traditionally shown on weather maps in blue. Compare the hand-twist model of a High to the wind directions in the several station area from the eastern Great Lakes to North Dakota about the southern side of this high-pressure center. Wind directions at stations across this area show that, as seen from above, the air spiraled generally [(clockwise) (counterclockwise)] around this high-pressure center, denoted by your H.
This wind flow pattern about the High is [(consistent with) (contrary to)] the hand-twist model of a High.
Based on the portions of station circles shaded to denote sky coverage, the skies in the broad area about the High were generally [(clear) (cloudy)].
The hand-twist model of a High includes vertical motions with air sinking. Based on the High shown on this map, areas of sinking air are likely to be locations of [(clear) (cloudy)] skies.
This pattern of cloud cover [(was) (was not)] consistent with high-pressure systems being "fair" implying clear skies and therefore, no precipitation.
One tool for wind speed conversions between miles per hour and knots (as well as other quantities) and their formulae can be found at: http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml. Also, common NWS terminology for regions of the country is at: http://www.hpc.ncep.noaa.gov/images/us_bndrys1_print.gif and http://www.hpc.ncep.noaa.gov/images/us_bndrys2_print.gif.
Suggestions for further activities: Try your own detective game. You can call up the AMS Weather Studies "U.S. Data" map and apply the hand-twist model to find High or Low centers. Then verify your positions by looking at the "Isobars, Fronts, Radar & Data" map.
Critical Thinking and Diversity Component: AMS Weather Studies is particularly suited to modeling critical thinking. In each Week's "A" Current Weather Studies we will alert you to the week's Critical Thinking and Diversity Component, which is located in the Learning Files section of the course homepage under Critical Thinking/Diversity. The Week 1 Critical Thinking/Diversity Component includes an introduction to critical thinking, a critical thinking activity, and an explanation of the diversity component.
If directed by your instructor, place the answers to Investigation 1A and Current Weather Studies 1A on the A Answer Form linked from the AMS Weather Studies website.
Return to AMS Weather Studies website
©Copyright 2008, American Meteorological Society
