Air pressure decreases as altitude increases.

Try the barometric pressure calculator.

Air pressure can be measured using a barometer.  There are two major types of barometers: liquid and aneroid.  The aneroid barometer does not use a liquid to measure pressure.  An example of a liquid barometer is the mercury barometer.  Air pushes down on the mercury.  The pressure causes the mercury to rise up the vacuum tube.  The tube is calibrated in inches or millimeters. 

Barometric Pressure Units

1 atmosphere = 760 mmHg = 29.92 inHg = 14.7 lb/in2 = 101.3 KPa

(kPa stands for kilopascals, where a pascal is the basic SI unit of pressure)

For weather applications, the standard atmospheric pressure is often called 1 bar or 1000 millibars (mb). This is convenient for recording the relatively small deviations from standard atmospheric pressure with normal weather patterns.