Air density is defined as the mass of air per unit volume. It is a measure of the number of air
molecules in a unit volume of air. Air is held to the earth’s surface by the force of gravity and
so, the higher one goes in the atmosphere, the ‘thinner’ (less dense) the air. An International
Standard Atmosphere (ISA) has been defined as a measuring stick against which the actual
atmosphere existing at a particular time and place can be compared. A number of characteristics
(such as pressure, temperature and density) are specified for various levels in the International
Standard Atmosphere.
• Air pressure in the ISA is 1013 hectoPascals at Mean Sea Level and the pressure
decreases with altitude, at about 1 hPa per 30 feet gain in height in the lower levels of
the atmosphere.
• Air temperature in the ISA is +15oC at Mean Sea Level and decreases at approximately
2°C per 1000 feet gain in altitude.
• Air density in the ISA decreases with a gain in altitude.
The main function of the Standard Atmosphere is to calibrate altimeters (which are basically
pressure reading devices) so that they match up certain air pressures with the correct altitudes.
With the altimeter set on 1013 hPa (ISA MSL), an altimeter will display a height that
corresponds to an altitude in the International Standard Atmosphere. This is known as Pressure
Height.
Fig 1. With standard pressure 1013 set in the sub-scale window an altimeter reads pressure
height
Actual Mean Sea Level Pressure Varies
The actual air pressure that exists at a given place varies from day to day and from hour to
hour. In aviation, we cope with this by using the QNH pressure setting in the altimeter sub-
scale that relates the altimeter reading to the sea level pressure, whatever it happens to be at
that time and place.
For the altimeter to read altitude (height above sea level) accurately, you must ensure that the
correct QNH is set in the sub-scale.
Calculating Pressure and Density Altitude
