Air pressure is the result of the weight of the column of air above the surface.
Air has mass, therefore it has weight in a gravity field.
At sea level a column of air one square centimeter in area exerts a pressure equal to 1 kilogram or
This is equal to 15 pounds/in2
Barometers measure the pressure exerted by the column of air above it.
The more air above the barometer, the more pressure.
The first barometer was a mercury barometer, invented by Evangelista Torricelli, a student of Galileo.
A long glass tube, closed at one end and filled with mercury, has theopen end immersed in a vat of mercury. The height of the mercury in the tube indicates the air pressure exerted on the mercury in the vat.
Why was mercury used?
A small canister with a partial vacuum will expand or collapse with changing pressure; this movement can be measured
Standards of Measurement
Inches of Mercury - in. Hg
- English System
- Standard sea-level pressure = 29.92 in. Hg
Centimeters of Mercury - cm Hg
- Metric System
- Standard sea-level pressure = 76 cm Hg
- Metric unit of pressure
- Standard sea-level pressure = 1 bar; 1 bar = 1000 millibars (mb)
Normal Pressure Variation
Normal barometric pressure typically range from 980 to 1030 mb or 28.9 to 30.4 in. Hg.
Air Pressure and Altitude
The density of the atmosphere decreases with altitude.
Pressure also decrease with increasing altitude.
Air pressure is not equal over the surface of the Earth.
Air pressure varies with changes in altitude (different locations are at different altitudes)
Air pressure also changes due to differing air masses (more air in one location resulting in a greater column of air).
Pressure changes from one location to another is called the pressure gradient.
Maps showing pressure changes from one location to another use isobars.
Isobars are lines of equal pressure.