Highs and Lows, with their comings and goings, are our major weathermakers. Being in the paths of these migrating weather systems, local measurements of air pressure often can be our major forecasting guide to the weather heading our way. Changes in air pressure due to passing weather systems are not great compared to the total air pressure. These changes are tens of millibars at most within a range from about 970 to 1040 mb, corrected to sea level. This change is only a few percent of the total pressure, a small amount compared to the change going straight up, where a one millibar drop occurs for approximately every 10 meter ascent in the lower atmosphere! But the change in pressure along the horizontal, and the distances and times involved still create strong storms such as the low pressure system in New England on Sunday night. On the other hand, small horizontal pressure changes over large distances are associated with expansive fair-weather systems, such as the large high located over the central Plains on Monday.
Atmospheric pressure readings, corrected to sea level, are reported by the National Weather Service each hour with changes over three-hour intervals (0Z to 3Z, etc.), also listed as a "pressure tendency". The DataStreme Atmosphere meteogram (short for meteorogram) that can be obtained for selected cities from the RealTime Weather Portal provides a graphical display of the hourly weather conditions at some station over the past 24 hours. The bottom panel of the chart is a plot of the time series of sea-level pressure readings, similar to the trace provided by a barograph, such as that found in Figure 5.6 of the text. Falling pressure values usually denote approaching storms (a low or front), while rising values accompany clearing or continued fair weather associated with an approaching high-pressure cell.
These changes have long been the basis of forecasting when based purely on local observations, "when the glass (barometer) is low, expect a blow"!
For more details describing variations in pressure with time, see the Supplemental Information…In Greater Depth below.
To be submitted on the lines for Tuesday on the Investigations Manual, Week 5 Chapter Progress Response Form.
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 of 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 Investigation 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. 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 meteorogram appearing on page 5A-6.) Pressure changes may be as much as 4 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 in the Weather Studies text. 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.