Altimeter and temperature
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.
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 Apr 2013 at 14:01.
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Great replies and help - thanks pruners
I'm comfortable with the concept of the column of air expanding so in flight, flying from an area of lower temp to higher temp will cause the altimeter to give a lower reading than your actual altitude i.e. your plane will be HIGHER than the indicated altitude.
However, there seems to be some disagreement on temperature and pressure when on the ground. If I'm on the ground (say airfield elevation is 1000ft) and I set my altimeter for airfield elevation. An hour later, the temperature has increased by 25 degrees (I have not changed the altimeter setting) - will my altimeter read higher or lower than before?? My assumption was that it would read higher, temp increased, air pressure decreased, higher reading.
Once again - appreciate the wealth of knowledge and replies........
I'm comfortable with the concept of the column of air expanding so in flight, flying from an area of lower temp to higher temp will cause the altimeter to give a lower reading than your actual altitude i.e. your plane will be HIGHER than the indicated altitude.
However, there seems to be some disagreement on temperature and pressure when on the ground. If I'm on the ground (say airfield elevation is 1000ft) and I set my altimeter for airfield elevation. An hour later, the temperature has increased by 25 degrees (I have not changed the altimeter setting) - will my altimeter read higher or lower than before?? My assumption was that it would read higher, temp increased, air pressure decreased, higher reading.
Once again - appreciate the wealth of knowledge and replies........
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However, there seems to be some disagreement on temperature and pressure when on the ground. If I'm on the ground (say airfield elevation is 1000ft) and I set my altimeter for airfield elevation. An hour later, the temperature has increased by 25 degrees (I have not changed the altimeter setting) - will my altimeter read higher or lower than before?? My assumption was that it would read higher, temp increased, air pressure decreased, higher reading.
NO, then the altimeter reads the same as before.
YES, then the altimeter changed, which way did the PRESSURE go?
UP, then the altimeter now reads lower than before.
DOWN, then the altimeter reads higher than before. (high to low, look out below, your altimeter reading will send you lower)
Can the PRESSURE go UP when the TEMPERATURE goes UP?
Yes.
Can the PRESSURE go DOWN when the TEMPERATURE goes UP?
Yes.
PRESSURE is NOT related to TEMPERATURE exclusively.
Last edited by darkroomsource; 11th Apr 2013 at 16:07.
Understand your sentiments Lightning Male 
, but I'll try again....
Speedbird.
You keep saying:
I and others keep telling you that is wrong, but for some reason you don't seem to grasp or accept this.
Here's 3 things to think about.
1. If higher temperatures equals lower pressures as you keep insisting (and presumably lower temperatures = higher pressures), how come pressure reduces with altitude (where both temperature and pressure, of course, reduce with altitude.) How could that be so if you were correct?
2. Here's some historic average surface pressure charts Note that the pressures are higher in the warmer areas (i.e. Southern England) than the cooler areas (i.e. Northern Scotland) and are higher in Summer than Winter. How could that be so if you were correct?
3. If pressure in a hot air ballon was lower than the surrounding air, it would collapse, so clearly its not. How could that be so if you were correct?
I'm happy to answer any further queries provided you don't include
within the question 
, or I'll be forced to respond with a somewhat rude 
, but I'll try again....Speedbird.
You keep saying:
Increased temperature results in lower pressure air
Here's 3 things to think about.
1. If higher temperatures equals lower pressures as you keep insisting (and presumably lower temperatures = higher pressures), how come pressure reduces with altitude (where both temperature and pressure, of course, reduce with altitude.) How could that be so if you were correct?
2. Here's some historic average surface pressure charts Note that the pressures are higher in the warmer areas (i.e. Southern England) than the cooler areas (i.e. Northern Scotland) and are higher in Summer than Winter. How could that be so if you were correct?
3. If pressure in a hot air ballon was lower than the surrounding air, it would collapse, so clearly its not. How could that be so if you were correct?
I'm happy to answer any further queries provided you don't include
Increased temperature results in lower pressure air
, or I'll be forced to respond with a somewhat rude
Last edited by Mariner9; 11th Apr 2013 at 19:15.
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Just back from travels. Thanks for replies. Apologies to those who seem to have got their feathers ruffled.
Mariner9 - the first reply to my post was in agreement with my temperature/pressure query, hence the comment.
I guess I assumed pressure was lower at higher temperatures because I had read that as temperature increases takeoff distances/engine performance and/or is equivalent to being at a HIGHER altitude (which I incorrectly connected to lower pressure).
All is clear now - thanks guys.
Took the test - got 59/60 (and no, was not the temp/altimeter Q I got wrong
)
Mariner9 - the first reply to my post was in agreement with my temperature/pressure query, hence the comment.
I guess I assumed pressure was lower at higher temperatures because I had read that as temperature increases takeoff distances/engine performance and/or is equivalent to being at a HIGHER altitude (which I incorrectly connected to lower pressure).
All is clear now - thanks guys.
Took the test - got 59/60 (and no, was not the temp/altimeter Q I got wrong
)
Last edited by Speedbird777; 18th Apr 2013 at 20:29.
