From: Subject: Tuesday Weather Topic in Greater Depth Date: Thu, 22 Oct 2009 10:28:14 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w05t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 6 = October 2009=20

INTERPRETING PRESSURE TENDENCY

The "pressure tendency" is the pressure change that occurs over a = given time=20 interval at a given locale. According to observational practice, the = pressure=20 tendency includes both the amount that air pressure changes together = with the=20 direction of change (increase or decrease) that has occurred over a = 3-hour=20 interval up to the present observation time. Inspection of the pressure = tendency=20 at any given location allows us to explain some of the changes in = weather that=20 may be occurring, and ultimately, helps in short-term forecasting of = future=20 weather events of the next several hours.

CAUSES OF PRESSURE CHANGES

Variations in pressure over time at a particular point are caused by = several=20 factors.

The movement of pressure systems

One factor to be considered is the movement of large-scale = atmospheric=20 pressure systems. Organized mid-latitude low pressure and high-pressure = systems=20 appear to move across the country on a sequence of surface weather = analyses. Let=20 us assume that the central pressure within the system did not change as = the=20 system moved. As these weather systems pass the vicinity of the station, = the=20 barograph will record these pressure changes:

When a migratory low pressure system approaches the observing = station, the=20 air pressure will fall with time, reaching a relative minimum as the = region of=20 lowest pressure passes over the station. As the low moves away, the = pressure=20 rises with time. Conversely, an approaching high-pressure system will = cause a=20 rise in the pressure with time, reach a relative maximum when the high = is=20 overhead, and then fall as the high moves away.

These variations in air pressure with time have been well known since = the=20 17th century, and have been used frequently as a means of = making a=20 short term, single-station forecast. Specifically, "falling" pressure = signals a=20 possible onset of stormy weather typically associated with an = approaching low=20 pressure cell, while "rising" pressure would usually suggest that "fair" = weather=20 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=20 time interval depends upon the speed that the pressure cell has moved = over that=20 time interval and upon the relative pressure difference between the = central=20 pressure of the approaching cell in question and that of the departing = system.=20 In other words, a rapid pressure fall over time can result from either a = fast=20 moving, but weak, low pressure system, or from an intense low pressure = system=20 that may move more slowly and that has a relatively "deep" central = pressure.=20

Frontal passages, especially those associated with cold fronts, may = be noted=20 for rapid pressure changes. Attention is directed to Investigation = 5A.=20 When a front is drawn on a surface weather analysis, its location is = typically=20 found near a trough of relatively low pressure as depicted by the isobar = analysis. (On the map appearing on page 5A-5 of Investigation 5A, = the=20 cold front extending southward toward north Texas from the surface low = in Lower=20 Michigan runs through the kinks in the isobars.) As the front = approaches, the=20 pressure at a point would fall until frontal passage, then start to = increase=20 after the front has passed the observer. Some intense cold fronts may = have=20 especially dramatic pressure changes, especially after the front passes = and the=20 cold, dense air mass that trails the front invades the area. (Inspect = the=20 meteorogram appearing on page 5A-7.) Pressure changes may be as much as = 4 mb in=20 an hour.

The life cycle of pressure systems

Another factor to be considered in identifying observed pressure = changes at a=20 station is the change in the central pressure of surface weather systems = over=20 time, regardless of the movement of the system. Suppose that the low = were=20 stationary. A low may "deepen" then "fill" with a corresponding pressure = fall,=20 followed by pressure rise. Correspondingly, a stationary high-pressure = system=20 could "build" with a pressure rise then weaken over several days. The = mechanisms=20 by which these systems intensify or weaken are identified and described = on page=20 138 of the Weather Studies text. Often times, the central = pressures=20 change with time as systems move across the earth's surface.

Typically, th e central pressures of many weather systems may change = by=20 several millibars over a three-hour interval. Some mid-latitude = low-pressure=20 systems, especially those over the open waters of the North Atlantic = Ocean,=20 develop rapidly. Meteorologists call those systems "bombs" when the = central=20 pressure falls by at least 24 mb in 24 hours.

A hurricane is an intense tropical low-pressure system. The central = pressures=20 of some of these systems have been observed to drop by more than 25 mb = in 12=20 hours during the intensification phase. When destructive hurricane = Andrew passed=20 over Miami, FL in 1992, the pressure trace indicated a fall of 6 mb in = an hour=20 before the arrival of the hurricane, followed by a 7-mb rise in an hour, = after=20 the central eye passed.

Return to the Tuesday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =