General Observations
- Low and High Air Pressure
- Low air pressure= less weight pushing down= air is rising and cooling which helps form clouds.
- High air pressure= more weight pushing down= air is sinking and warming with less humidity.
- Falling or Low Barometric Pressure
- Deteriorating weather or some form of precipitation such as wet and/or windy conditions and thunder in hot weather are associated with falling barometric pressure.
- As a storm approaches air pressure begins to fall and this can happen very quickly.
- Storms are associated with low air pressure.
- Rising or High Barometric Pressure
- As the storm leaves the air pressure will tend to rise gradually and is associated with approaching fairer weather.
- Rising barometric pressure is a good indicator that no storms are developing in the following 24 hrs and there will be fair weather or no precipitation.
- In winter, rising pressure can also indicate snow if the weather has been frosty.
- Clear skies are associated with high air pressure.
- Localized Pressure
- Localized low pressure represents the path of least resistance for a weather system and is associated with increased storm activity.
- Localized high pressure acts as a barrier to approaching weather systems, diverting their course.
- Weather Fronts and Isobars
- The boundary between a system of warm and cold air is known as a ‘front’ which coincides with changes in air pressure.
- Weather maps (surface chart) and forecasts usually show areas of low (red “L”) and high pressure (blue “H”) as well as weather fronts.
- Isobars, lines of equal pressure, when drawn on such a map, give a contour map showing areas of high and low pressure.
Range of Barometric Pressure on Earth
Refer to Barometer Calibration
Average Sea-Level pressure:
101.325 kPa /1013.25 mbar / 760 mmHg / 29.921 inHg
Pressures seldom increase or decrease 1 inch of mercury ( 3.386 kPa /33.86 mbar / 25 mmHg) above or below the 30 inch mark ( 101.591kPa / 1015.91 mbar / 762 mmHg) unless weather conditions are extreme.
Typical readings that would provide a reasonable forecast for 12-24 hrs ahead:
| READING | RISING OR STEADY | SLOWLY FALLING | RAPIDLY FALLING |
|---|---|---|---|
| OVER 102.269 kPa /1022.69 mbar /767 mmHg /30.20 inHg |
Continued fair | Fair | Cloudy, Warmer |
| BETWEEN 102.269 kPa /1022.69 mbar /767 mmHg /30.20 inHg AND 100.914 kPa / 1009.14 mbar/757 mmHg / 29.80 inHg |
Same as present | Little change | Precipitation likely |
| UNDER 100.914 kPa / 1009.14 mbar /757 mmHg / 29.80 inHg |
Clearing, cooler | Precipitation | Storm |
Extreme examples of barometric pressure:
| EVENT | DATE | LOCATION | CONDITIONS |
|---|---|---|---|
| Lowest ever recorded pressure on Earth 87kPa /870 mbar / 652.53 mmHg /25.69 inHg |
12 October 1979 | Typhoon Tip, W. Pacific Ocean |
Largest, most intense tropical cyclone on record Peak width: 2,200 km / 1,380 miles Peak winds: 305 km/h or 190 mph |
| Highest ever recorded pressure on Earth
Most common official citation: Alternative citation that is currently uncertified : |
31 December 1968
19 December 2001 |
Agata Lake, N. Siberia
Tosontsengel, Mongolia
|
Clear and extreme cold -46C/-50.8F
Clear and extreme cold |