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AIR PRESSURE AND WIND |
Note: This Activity is the same as AMS Weather Studies Current Weather Studies 1B from Preview Week. Students only need to complete the Activity once.
The Image 1 "U.S. - Data" map acquired from the course website is the depiction of weather conditions at stations across the contiguous U.S. at 00Z 30 AUG 2009. [00Z is four hours ahead of Eastern Daylight Time (EDT), so the map depicts conditions at 8 PM EDT (7 PM CDT, 6 PM MDT and 5 PM PDT) on Saturday evening.] The general weather conditions across the U.S. at the time of the map were storminess from the Great Lakes to the Northeast and along the Eastern Seaboard while the north central portion of the country had relatively cool, fair weather. Other showers were scattered across the southern and western parts of the contiguous U.S.
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 Glasgow, in northeastern Montana, at map time was from the [(northeast)(northwest)(southeast)(southwest)].
(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 Glasgow was from, it would be reported as a [(northeast)(northwest)(southeast)(southwest)] 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 course website).] At map time, Glasgow had a 15-knot wind (one long and one short feather). [A double circle without a direction shaft signifies calm conditions, such as Spokane in eastern Washington state and Phoenix, AZ, and a shaft without feathers denotes 1-2 knots. One knot (nautical mile per hour) is about 1.2 land (statute) miles per hour.]
A bold red "L" and a bold blue "H" have already been marked on the map. Compare the hand-twist model of a Low to the wind directions in the several state area about this low-pressure center. Wind directions at stations across this area from the Great Lakes to eastern New York state show that, as seen from above, the air spiraled generally [(clockwise)(counterclockwise)] around this low-pressure center, denoted by the L. With strong, well-developed Lows, the air also spirals inward toward 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 several state 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 characterized as "stormy", implying extensive cloudiness and possibly precipitation.
Wind directions at stations from Minnesota through the Dakotas to eastern Montana show that, as seen from above, the air spiraled generally [(clockwise)(counterclockwise)] around the high-pressure center, denoted by the H in south-central Canada. With strong, well-developed Highs, the air also spirals outward from the high-pressure center.
The hand-twist model of a High includes vertical motions with air sinking. Based on the High on this map, areas of sinking air are likely to be locations of generally [(clear)(cloudy)] skies.
The general pattern of cloud cover in the north-central U.S. associated with the H [(was)(was not)] consistent with high-pressure systems being "fair" implying clear skies and therefore, no precipitation.
Look at the Image 2 map from the Monday Current Weather Study A. This surface weather map [(was)(was not)] for the same day and time as the U.S. - Data map of this activity.
Note the precipitation areas as denoted by the radar echoes on the Image 2 map of the Monday Current Weather Study. The precipitation areas about the Northeastern U.S. [(do)(do not)] further support the indication of the Low as a "stormy" weather system.
When the the current weather map available on the AMS Weather Studies website shows centers of stormy Lows or fair weather Highs near your location, you might try to fit your local wind direction (as shown by a flag, for example) with map circulations and the hand-twist model of weather systems. The designation of the Ls and Hs as centers of stormy and fair weather systems, respectively, can be compared to satellite views showing clouds across the U.S. Check to see if the region immediately around an L is generally cloudy or the broad area centered on an H as mostly clear.
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.
If directed by your instructor, place the answers to Investigation 1B and Current Weather Studies 1B on the B Answer Form linked from the AMS Weather Studies website.
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