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. They 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 approaching the Pacific Northwest on Monday. On the other hand, small horizontal pressure changes over large distances are associated with the expansive fair-weather systems as enjoyed by many locales in the Midwest over the past weekend.
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 meteogram (short for meteorogram) that can be obtained for selected cities from the DataStreme Homepage, 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 5.4 of Part A. 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 Tuesday's optional electronic Supplemental Information .
To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 5 Chapter Progress Response Form, under section B. Daily Summary.