        SUPPLEMENTARY INFORMATION: INTERPRETING PRESSURE TENDENCY

        This Supplemental Information File complements the Daily 
        Summary of Tuesday, 15 October 1996

        The "pressure tendency" describes 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.  Variations in pressure over time at 
        a particular point are caused by several factors.

        ** The movement of pressure systems
        One factor to be considered is the movement of large scale 
        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 pressure will fall with time, reaching a relative 
        minimum as the region of lowest pressure passes over the 
        station, then 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 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 found on a surface weather 
        analysis, it 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.

        ** The life cycle of pressure systems
        Another factor to be considered in identifying observed 
        pressure changes at a station is the change in the central 
        pressure of surface pressure features over time, regardless of 
        the movement of the system.  A low may "deepen" then "fill" 
        with a corresponding pressure fall, then pressure rise, while a 
        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.

        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 pressure of some of these systems has been observed to 
        drop by more than 25 mb in 12 hours during the intensification 
        phase.  When the destructive hurricane Andrew passed over Miami 
        in 1992, the pressure trace at Miami, FL indicated a fall of 6 
        mb in an hour before the arrival of the hurricane, followed by 
        a 7 mb rise in a hour, after the central eye passed.

        --   Filename=f96wk05t_sup.txt

        Prepared by Edward J. Hopkins, Ph.D.
        (c) Copyright, 1996, The American Meteorological Society.
