Download NAVEDTRA 14342 Air Traffic Controller

Atmospheric Pressure and Temperature
Background
The atmosphere is made up of molecules that we call air. These air molecules
have weight (approximately 1.2 ounces per cubic foot at sea level), and the
amount of weight these air molecules exert on the earth's surface is called
atmospheric pressure. You must understand how an aircraft uses atmospheric
pressure to determine altitude and how pressure and temperature changes have
an affect on the aircraft's instruments.
Units of
measurement
There are two basic units used to measure and report the atmospheric pressure,
inches and millibars.
Atmospheric pressure is measured using either a mercury or aneroid barometer.
Air pressure pressing against a mercury barometer causes mercury to rise in an
evacuated glass tube. Air pressure at sea level causes the mercury to rise in the
glass tube, on the average, 29.92 inches (standard sea-level pressure). Mercury
is used because it is such a dense, heavy liquid, the same pressure would cause
water to rise approximately 400 inches in the same tube. An aneroid barometer
uses a thin metal strip in an evacuated case to measure pressure.
In the United States, we report the barometric pressure in inches (for example,
29.92 inches) and this is the unit of measurement that you will be most
concerned with. However, a pilot will occasionally ask for the altimeter in
millibars which is the scientific unit of measurement. Normally you will have
to contact the weather reporting service for your station to obtain this reading.
The table below gives a comparison of inches to millibars:
Inches
Millibars
Inches
Millibars
26.58
900
28.94
980
26.87
910
29.24
990
27.17
920
29.53
1000
27.46
930
29.82
1010
27.76
940
29.92
1013.25
28.05
950
30.12
1020
28.35
960
30.42
1030
28.64
970
30.71
1040
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