The static pressure in still air at any point in the atmosphere is determined by the mass of the column of air above that point. Increasing temperature in a column of air causes the air to expand. The earth prevents the column from expanding downwards and the surrounding columns limit the horizontal expansion, so the majority of the expansion occurs in the upward direction. This upward expansion is illustrated in the diagram provided by Lightning Mate.
This upward expansion of the column of air does not change the mass of air in the column, but it causes a greater proportion of that air to lie above any given point above the surface. Because the total mass of air in the column does not change, the pressure at ground level does not change. But because more of the mass lies above any point above ground level, the pressure at that point increases. These effects can also be seen the diagram provided by Lightning Mate.
The increase in static pressure at any point above the ground decreases the altimeter reading. So increasing air temperature decreases the indicated altitude at any given point above the surface.
If temperature of the column is decreased these process are reversed, causing the column to contract down towards the earth. This causes the pressure at any point above the surface to decrease, which in turn causes the altimeter to over indicate. This means that barometric altimeters under-read when air temperature is greater than ISA and over-read when temperature is lower than ISA.
The indication error that is caused by this phenomenon can be calculated using the following formula:
Error = 4 ft per degree C ISA deviation per 1000 feet true altitude.
Note that from the above formula, when the true altitude is zero the error will be zero.
The atmosphere can of course produce other effects, some of which will change the surface pressure. But these surface pressure changes can be corrected for by adjusting the altimeter subscale.
Last edited by keith williams; 11th April 2013 at 14:01.