CWS 1B

SURFACE AIR PRESSURE PATTERNS

Do Now:

Print this file.
Print the associated Current Weather Studies B Image 1 and Image 2 Files.
Print the Daily Summary and/or Supplemental Information Files, if directed by your instructor.

To Complete Investigation:

Reference: Chapter 1 in the Weather Studies textbook.
Complete Investigation 1B in the Weather Studies Investigations Manual as directed by your instructor.
Complete this Wednesday online-delivered Current Weather Studies 1B activity if directed by your instructor.

Current Weather Studies Investigation 1A dealt with the hand-twist model for relating wind directions to centers of high or low air pressure. This Current Weather Studies Investigation 1B demonstrates how those air pressure patterns (Highs, Lows) are determined.

Image 1 ("Pressures" map) was acquired from the course website and reports surface air pressures (corrected to sea level) rounded to the nearest whole millibar at 12Z 25 AUG 2008. The lowest plotted air pressure on the map is _______ mb in both central Mississippi and northern Montana.
The highest reported pressure is 1023 mb in [(Denver, CO) (Duluth, MN)].
The isobars in the conventional series that will be needed to complete the pressure analysis between the lowest and highest values on this map are: 1004, _____, _____, _____, and _____. More than one isobar of the same value may need to be drawn if pressure values located in separate sections of the map area require it. The isobaric pattern across much of the western third of the U.S. has already been drawn.

Using a pencil, follow the steps below to complete the pressure analysis in the eastern two-thirds of the map to determine the pressure pattern that existed at the time the observations were made. Consider each pressure value to be located at the center of the reported number.

Arbitrarily we might start by drawing the 1004-mb isobar, the first of the series. Values of "1003" appear in two widely separated locations on the map so two 1004-mb isobars will be needed. The 1003-mb value in Montana already has a 1004-mb isobar drawn separating it from the neighboring higher values. The ends of the isobar extend just past the border and are labeled with its value. Consider next the "1003" located in Mississippi. Plotted pressure values surrounding this value are [(less than) (equal to) (greater than)] 1003. Therefore, a 1004-mb isobar would completely encircle the 1003-mb value. Label this isobar with its value by printing 1004 within a break of the circle.

Continue drawing and labeling isobars until all those of the sequence you listed above have been properly placed. Note that the 1008-mb isobar appears in more than one location on the map. As was shown across the West, label your isobars with their values at the ends that extend just beyond the data field. Complete your pressure analysis by placing a bold L where the lowest pressure value was enclosed by an isobar and a bold H in the center of Lake Superior representing the approximate location of the highest pressure value.

Image 2 is the "Isobars, Fronts, Radar & Data" map from the course website for 12Z 25 AUG 2008. The Image 2 map includes station models as well as computer-drawn isobars and radar echoes. The centers of high and low pressures as well as frontal systems are also shown. The Image 2, "Isobars, Fronts, Radar & Data" map [(is) (is not)] the same time and date as the Image 1 map of pressures you have just analyzed.

Compare your isobar pattern with that drawn on the Image 2 map. The Image 2 map is constructed by computer based on a much more complete set of pressure values. (This accounts for some of the variations between your analysis and that by the computers. It also is the source of several of the plotted Hs and Ls denoting locally minimally higher or lower pressure centers, respectively.)
The "U.S. Data" map examined in the Monday, Current Weather Studies 1A [(is) (is not)] the same time and date as your analyzed Image 1 Pressures map. The Monday map allowed you to practice the hand-twist model of Lows and Highs. Your analysis of the Image 1 Pressures map shows why we designated those pressure centers on the Monday map.
This Current Weather Studies 1B, Image 2 map also includes the centers of the lowest (L) and highest (H) pressures as determined by computer analysis using a more complete set of pressures. The L and H positions on this Image 2 map [(are) (are not)] in the same general locations as the L and H you positioned in your Image 1 map of analyzed pressures and on the Monday U.S. Data map used for your hand-twist models.
The wind directions reported in the Louisiana to Georgia Gulf area reveal a general [(clockwise) (counterclockwise)] circulation as expected from the hand-twist model of a Low. The radar shadings further display the semi-circular rain shield associated with Tropical Depression Fay at that time.

The wind directions reported over several U.S. states centered on Lake Superior show the circulation associated with the High and depicted by the hand-twist model.

By looking at these maps you can relate the reported wind patterns to the plotted centers of high and low air pressure. The radar echoes representing precipitation further confirm the "stormy" Low that was Fay while the "fair weather" High of the Great Lakes lacks them. The Week 2 Investigations will look at the more complete set of weather conditions reported in the station models including coded air pressure values.

For more practice on drawing isopleths (lines of a constant value) in groups of numbers, from simple to more complex patterns, go to: http://profhorn.aos.wisc.edu/wxwise/AckermanKnox/chap1/Contour_page1.html.

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.