The "pressure tendency" is the pressure change that occurs over a given time interval at a given locale. According to observational practice, the pressure tendency includes both the amount that air pressure changes together with the direction of change (increase or decrease) that has occurred over a 3 hour interval up to the present observation time. Inspection of the pressure tendency at any given location allows us to explain some of the changes in weather that may be occurring, and ultimately, helps in short-term forecasting future weather events of the next several hours.
Variations in pressure over time at a particular point are caused by several factors.
One factor to be considered is the movement of large scale atmospheric pressure systems. Organized mid-latitude low pressure and high pressure systems appear to move across the country on a sequence of surface weather analyses. Let us assume that the central pressure within the system did not change as the system moved. As these weather systems pass the vicinity of the station, the barograph will record these pressure changes:
When a migratory low pressure system approaches the observing station, the air pressure will fall with time, reaching a relative minimum as the region of lowest pressure passes over the station. As the low moves away, the pressure rises with time. Conversely, an approaching high pressure system will cause a rise in the pressure with time, reach a relative maximum when the high is overhead, and then fall as the high moves away.
These variations in air pressure with time have been well known since the 17th century, and have been used frequently as a means of making a short term, single-station forecast. Specifically, "falling" pressure signals a possible onset of stormy weather typically associated with an approaching low pressure cell, while "rising" pressure would usually suggest that "fair" weather may occur that is often found with a high pressure cell.
If all other factors were equal, the amount of pressure change over a given time interval depends upon the speed that the pressure cell has moved over that time interval and upon the relative pressure difference between the central pressure of the approaching cell in question and that of the departing system. In other words, a rapid pressure fall over time can result from either a fast moving, but weak, low pressure system, or from an intense low pressure system that may move more slowly and that has a relatively "deep" central pressure.
Frontal passages, especially those associated with cold fronts, may be noted for rapid pressure changes. Attention is directed to Activity 5A. When a front is drawn on a surface weather analysis, its location is typically found near a trough of relatively low pressure as depicted by the isobar analysis. (On the map appearing on page 5A-2 of Activity 5A, the cold front extending southward toward Alabama from the surface low in northern Indiana runs through the kinks in the isobars.) As the front approaches, the pressure at a point would fall until frontal passage, then start to increase after the front has passed the observer. Some intense cold fronts may have especially dramatic pressure changes, especially after the front passes and the cold, dense air mass that trails the front invades the area. (Inspect the meteorograms appearing on page 5A-3.) Pressure changes may be as much as 6 mb in an hour.
Another factor to be considered in identifying observed pressure changes at a station is the change in the central pressure of surface weather systems over time, regardless of the movement of the system. Suppose that the low were stationary. A low may "deepen" then "fill" with a corresponding pressure fall, followed by pressure rise. Correspondingly, a stationary high pressure system could "build" with a pressure rise then weaken over several days. The mechanisms by which these systems intensify or weaken are identified and described on page 86 of Part A: Narrative. Often times, the central pressures change with time as the systems move across the earth's surface.
Typically, the central pressures of many weather systems may change by several millibars over a three hour interval. Some mid-latitude low pressure systems, especially those over the open waters of the North Atlantic Ocean, develop rapidly. Meteorologists call those systems "bombs" when the central pressure falls by at least 24 mb in 24 hours.
A hurricane is an intense tropical low pressure system. The central pressures of some of these systems have been observed to drop by more than 25 mb in 12 hours during the intensification phase. When destructive hurricane Andrew passed over Miami, FL in 1992, the pressure trace indicated a fall of 6 mb in an hour before the arrival of the hurricane, followed by a 7 mb rise in an hour, after the central eye passed.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.