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:
108.38kPa /1083.8 mbar / 812.92mmHg / 32.01 inHg


Alternative citation that is currently uncertified :
108.57 kPa/ 1085.7 mbar/ 814.32 mmHg /32.06 inHg


31 December 1968 




19 December 2001


 

Agata Lake, N. Siberia




Tosontsengel, Mongolia


Clear and extreme cold -46C/-50.8F  




Clear and extreme cold