WELCOME BACK: Procedure continued from STUDY GUIDE.
Last week, a powerful storm system moved across much of the country, producing a severe weather outbreak across the nation's midsection and snow in the northern Plains. As detailed in the Thursday, October 25, 2001, Daily Weather Summary, as many as 20 tornadoes were reported to the Storm Prediction Center from Missouri, Illinois, Indiana, Michigan, Ohio and Tennessee by early Thursday morning. On Wednesday, Grand Forks received 10.8 inches of snow, which set a daily snowfall record and the monthly record for October at that location. Image 1 is the surface weather map (Isobars, Fronts, Radar, & Data) for 12Z, Thursday, 25 OCT 2001, showing the locations of the low pressure center and associated fronts. At this time, the Low was centered over Lake Superior with a central pressure of 975 mb. This strong Low had a central pressure characteristic of minimal hurricanes!
The circulation pattern of winds about the western side of the Lake Superior low pressure center was generally [(clockwise and outward) (counterclockwise and inward)]. This patterns [(is) (is not)] consistent with that of the hand-twist model of a Low used earlier in Activity 1B.
Bismarck, North Dakota, has coded pressure digits of "203". This is decoded to a sea-level pressure of _________ mb. Minneapolis, Minnesota, has coded pressure digits of "996". This is decoded to a sea-level pressure of _________ mb. The pressure difference over this distance is __________ mb.
The horizontal pressure gradient force is the dominant force in bringing about large-scale horizontal air motions. The strength of the pressure gradient force, the pressure difference over distance, is shown by the isobar patterns. Between Bismarck and Minneapolis, counting the lines that cross the station models of each location, there are [(2) (3) (6)] isobar lines plotted. By comparison, there is only one isobar in the comparable distance between Witchita, Kansas, and Little Rock, Arkansas, where the pressure difference was _________ mb.
In general, the stronger pressure gradient, as shown by the number of isobars across a given distance, was located in the [(northern Plains and Great Lakes) (southern Plains and western Gulf Coast)] areas at map time. Several stations from Manitoba and Western Ontario, Canada around to Lake Michigan had reported wind speeds of 20 knots. Minneapolis reported 25 kts. Wind speeds across the Southern Plains and Western Gulf Coast areas ranged from calm to about 10 knots.
Based on the isobar pattern, the area of stronger pressure gradient on this map is [(Minnesota) (Oklahoma)]. In general, where isobars are closely spaced (strong pressure gradient), wind speeds are [(lower) (higher)].
Minneapolis, Minnesota, where it was 33 °F and overcast under the influence of the Lake Superior Low, reported a wind direction from the [(southeast) (west-northwest)]. The orientation of the isobar at Minneapolis indicates lowest pressure was generally in the direction towards the [(northeast) (southeast)]. The horizontal pressure gradient force acting on the air at Minneapolis was directed generally toward the center of low pressure, and [(perpendicular) (parallel)] to the isobars.
In the Northern Hemisphere, moving air also experiences a Coriolis force that is directed to the [(right) (left)] of the direction of motion. Consequently, the Coriolis force acting on the wind at Minneapolis would be directed toward the [(east-southeast) (south-southwest)]. The Coriolis force acting on the wind at St. Louis, Missouri would be directed toward the [(right) (left)] of the wind direction, to the southeast.
A third force acting on air moving near the Earth's surface is friction. Friction, which always acts in the direction opposite to motion, retards the movement of the air. The direction of the frictional force on the air at Minneapolis was towards the [(south-southeast) (west-northwest)]. The frictional force acting on the air at St. Louis was toward the [(northwest) (southwest)].
The Lake Superior Low moved northeastward into Canada on Friday. By Friday evening the low was located over central Quebec and a high pressure system had moved from Central Canada into the Northern Plains, bringing colder air to the eastern half of the United States. Image 2 is the surface weather map (Isobars, Fronts, Radar, & Data) for 00Z, Saturday, 27 OCT 2001. The circulation pattern of winds about the northern Plains High was generally [(clockwise and outward) (counterclockwise and inward)].
At Minneapolis by 00Z, Saturday, 27 OCT 2001, the central pressure had risen to _________ mb and the wind speed had fallen to about _________ kts. The pressure gradient force at Minneapolis was directed generally toward the [(east) (west)]. The Coriolis Force was directed toward the [(northeast) (southwest)] and the frictional force was toward the [(northwest) (southeast)]. These three forces acted together to produce a wind at Minneapolis from the northwest.
The frictional force, having slowed the air and reduced the Coriolis force, brings about the flow of air along a path cutting across the isobars towards lower pressure. In this way, air spirals into lows and out of highs as shown with the WeatherCycler and the hand-twist model.
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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