Physical Geography 101
Weight of column of air above 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
Devices measuring pressure exerted by column of air.
More air above barometer = greater pressure.
First barometer: mercury barometer
Invented by Evangelista Torricelli, a student of Galileo.
Long glass tube, closed at one end and filled with mercury; Open end immersed in vat of mercury. The
Height of mercury in tube indicates air pressure exerted on mercury in vat.
Why was mercury used?
Aneroid Barometer: small canister with partial vacuum will expand or collapse with changing pressure.
Capacitive Resistance Barometers: Two small plates in electric circuit move closer or farther apart, changing electrical resistance.
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)
- Actual sea-level pressure = 1013.2 mb
- 100 Pa = 1 hectopascals = 1 mb
Air Pressure and Altitude
Normal barometric pressure range: 980 to 1030 mb or 28.9 to 30.4 in. Hg.
Pressure decreases with increasing altitude.
Air pressure is not equal over Earth’s surface, due to:
Changes in altitude
Differing air masses (more air in one location).
Pressure changes from one location to another.
Isobar Maps: show pressure changes from one location with lines of equal pressure.