Atmospheric pressure is measured with an aneroid barometer that expands or contracts as atmospheric pressure changes.

Atmospheric pressure is a key parameter used for weather forecasting. Measuring the atmospheric pressure helps to locate surface troughs, high pressure systems, and frontal boundaries.


The average (or standard) pressure at sea level measures 29.29 inches of mercury.

Atmospheric pressure decreases 4% per 1000 feet of elevation; it is only about 21 inches of mercury at an elevation of 10,000 feet.

Meteorologists convert the measured atmospheric pressure at a weather station, such as Medford (1300 feet above sea level), to its equivalent sea level value.

Unit of measure = inches of mercury


If the barometer begins a steep and rapid descent, there is a good chance a storm is approaching; the faster the barometer falls, the windier it will be.

x axis is day of month


A rapidly falling barometer can indicate an approaching summer rainstorm.

This is a good time to check the weather forecast to see if you might be able to shut off your irrigation for awhile.

A summer rainstorm is also a good opportunity to have your rain catchment system refilled. Check to be sure it’s in good working order.

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