Montana Earth Science Picture of the Week

Counterclockwise Flow Around a Mid-Latitude Low

Map Courtesy of Atmospheric Sciences Department, University of Illinois at Urbana-Champaign

One very windy night in Montana . . .
A storm system swept through Montana during October of 2003, bringing the state its first taste of winter weather. The storm was a type that textbooks call "mid-latitude cyclones" or "mid-latitude lows". TV meteorologists refer to them as "low pressure systems" or simply "storm systems", perhaps because the word "cyclone" sounds too frightening. The use of the word "cyclone" indicates that the storm has winds that spiral in a counterclockwise direction toward a center of low pressure (clockwise in the Southern Hemisphere). Many of these low pressure systems cross the state each year, especially in the fall and spring. The type of weather they bring depends on the amount of water vapor in the air, as well as pressure and temperature differences within the system.

The bull's eye tells the story . . .
Although the storm brought some snow and much colder temperatures, one of its most extreme characteristics was its strong wind. Much of Montana experienced wind speeds over 50 miles per hour as the storm crossed the state during the early morning hours. The white lines on map show that the storm was centered on the Montana-Wyoming border at 5 am MST (12 Z) on October 29, 2003. These lines, called isobars, are lines of equal pressure, and in this case they make a huge bull's eye extending from Texas to Canada with the pressure decreasing toward the center (L). Locations on the isobar closest to the L were experiencing a pressure of only 988 millibars (29.2 inches of mercury). The pressure was low because this was the center of region of rising air. The spacing of the isobars also tells that the winds associated with the storm were exceptionally strong. Isobars that are close together indicate a bigger difference in pressure over distance (pressure gradient). It was this big difference in pressure that caused the strong winds. Based on the spacing of the lines, winds at the time the map was made were stronger in northeastern Montana than in the central part of the state.

Check it out - Animation: low pressure system centered over Texas on March 22, 2012

Little green arrows . . .
The green arrows, which show wind direction, reveal the distinct counter clockwise flow around the center of the storm. At the time the map was made (5 am MST), the winds in northeastern Montana were out of the east, whereas the winds in the southwestern part of the state were out of the west. During the night, as the storm's center swept across from Idaho to Wyoming, the wind at each location was determined by where the storm's center was. For example, earlier in the night, central Montana would have experienced winds out of the southwest. Then, as the storm's center passed through, the winds in central Montana would have started to blow out of the northwest.

Watch this Animation of a mid-latitude cyclone..

Term: pressure gradient


MUST SEE! Animation of Current Winds in USA
current maps like the one shown above
*The difference between Highs and Lows
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By Rod Benson
Earth Science Teacher at Helena High School

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