# Meteorology 106 Air Pressure

### Air Pressure

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

### Barometers

Barometers measure the pressure exerted by the column of air above it.
The more air above the barometer, the more pressure.

### Mercury Barometer

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?

### Aneroid Barometer

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

Bar
- 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.