Physical Geography 101
Atmospheric Pressure


Air Pressure

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
1 kilogram/cm2
This is equal to 15 pounds/in2

Barometers

Devices measuring pressure exerted by column of air.
More air above barometer = greater pressure.

Mercury Barometer

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?

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Other Barometers

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

Bar
- Metric unit of pressure
- Standard sea-level pressure = 1 bar; 1 bar = 1000 millibars (mb)
- Actual sea-level pressure = 1013.2 mb

Pascal (SI)
- 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.

Pressure Gradients

Air pressure is not equal over Earth’s surface, due to:
Changes in altitude
Differing air masses (more air in one location).

Pressure Gradients

Pressure changes from one location to another.
Isobar Maps: show pressure changes from one location with lines of equal pressure.