Will it rain when barometric pressure drops?

Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather because weather patterns are carried around in regions of high and low pressure (see Air Mass). A slowly rising atmospheric pressure, over a week or two, typically indicates settled weather that will last a long time. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, which will not last long, with heavy rain and strong winds. Pioneering meteorologist Vice-Admiral Robert Fitzroy once noted: "long foretold, long last; short notice, soon past".

By carefully watching the pressure on a barometer, you can forecast local weather using these simple guidelines:

  • Decreasing barometric pressure indicates storms, rain and windy weather.
  • Rising barometric pressure indicates fair, dry, and colder weather.
  • Slow, regular and moderate falls in pressure suggest a low pressure area is passing some distance away. Marked changes in the weather are unlikely.
  • Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by short lasting wind and showers.
  • A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.
  • Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop.
  • A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.
  • Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.
  • A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.

For pressure tendency, the definitions used by these guidelines are:

TermPressure change

over 3 hourshPain-Hgmm-HgSteady Less than 0.1 0.003 0.08 Slowly rising or falling 0.15 to 1.5 0.003 to 0.04 0.08 to 1.1 Rising or falling 1.6 to 3.5 0.05 to 0.1 1.2 to 2.6 Quickly rising or falling 3.6 to 6.0 0.1 to 0.18 2.7 to 4.5 Rapidly rising or falling More than 6.0 0.18 4.5

Guidelines and table adapted from: Weather Doctor's Weather Eyes.

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