atmospheric pressure - Standard atmospheric pressure, Mean sea level pressure (MSLP or QFF), Altitude Atmospheric Pressure Variation
The pressure or force exerted by the atmosphere on the Earth's surface. The average pressure at sea level is taken as standard, and it is usual to modify measurements taken at different elevations so that they refer to this level. Pressure varies with elevation (decreases with altitude) and temperature. Units of measurement vary: at sea level, average pressure is 1013·25 mb (millibars, the units generally used by meteorologists), 101 325 Pa (pascals), 760 mm of mercury (760 torr), 29·92 in of mercury, or 14·7 lb/sq in. Pressure is measured using a barometer.
Atmospheric pressure is the pressure at any point in the Earth's atmosphere.
In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location.
Standard atmospheric pressure
Standard atmosphere (symbol: atm) is a Units of pressure and is defined as being precisely equal to 101,325 Pa. This value is intended to represent the mean sea level pressure at the latitude of Paris, France, and as a practical matter, approximates the mean sea level pressure for many of the industrialized nations (those with latitudes similar to Paris). (see also Standard temperature and pressure)
In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of air at standard temperature and pressure.
Mean sea level pressure (MSLP or QFF)
Mean sea level pressure (MSLP or QFF) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure.
The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone.
The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly. In Canada's public weather reports, sea level pressure is reported in kilopascals , while Environment Canada's standard unit of pressure is the same .) In the weather code, three digits are all that is needed, Decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132;
Altitude Atmospheric Pressure Variation
Pressure changes quickly but smoothly between the earth's surface and outerspace. To find the pressure (in kPa or % atm) at any altitude (in feet or meters), you can click here to use the eXtreme High Altitude Calculator
| fraction 1 atm | avg altitude in meters |
| 1 | 0 |
| 1/2 | 5486.3 |
| 1/3 | 8375.8 |
| 1/10 | 16131.9 |
| 1/100 | 30900.9 |
| 1/1000 | 48467.2 |
| 1/10000 | 69463.6 |
| 1/100000 | 96281.6 |
Local Atmospheric Pressure variation
Atmospheric pressure varies widely on Earth, and these variations are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.
The highest recorded atmospheric pressure, 108.6 kPa (1086 mbar or 32.06 inches of mercury), occurred at Tosontsengel, Mongolia, 19 December 2001.
The lowest recorded non-tornadic atmospheric pressure, 87.0 kPa (870 mbar or 25.69 inHg), occurred in the Western Pacific during Typhoon Tip on 12 October 1979.
Atmospheric pressure shows a diurnal (twice-daily) cycle caused by global atmospheric tides.
Intuitive feeling for atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 mm (30 inches) of mercury is often used to teach, make visible, and illustrate (and measure) atmospheric pressure.
One atmosphere (101.325 kPa or 14.7 lbf/in²) is the amount of pressure that can lift water approximately 10.3 m (33.9 feet).
In terms of city water pressure, one atmosphere is approximately one-half to one-fifth the pressure of typical city water mains (i.e., water pressure is around 2 to 5 atmospheres).
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