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Temperature/Pressure - altitude

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For some reason this is not sticking in my head, and is not making logical sense to me and its driving me nuts


Variations in air temperature also affect the altimeter. On a warm day, the expanded air is lighter in weight per unit volume than on a cold day, and consequently the pressure levels are raised. For example, the pressure level where the altimeter indicates 10,000 feet will be HIGHER on a warm day than under standard conditions. On a cold day, the reverse is true, and the 10,000-foot level would be LOWER. The adjustment made by the pilot to compensate for nonstandard pressures does not compensate for nonstandard temperatures. Therefore, if terrain or obstacle clearance is a factor in the selection of a cruising altitude, particularly at higher altitudes, remember to anticipate that COLDER-THAN-STANDARD TEMPERATURE will place the aircraft LOWER than the altimeter indicates. Therefore, a higher altitude should be used to provide adequate terrain clearance.


Why is the bold statement not making sense to me!??!


In my head

Raise temp ==> lower pressure

air temperature is higher, and therefore the air particles are more spread apart = lower pressure.


higher temp --> Lower pressure would indicate on a altimeter as HIGHER than true altitude. So in reality my aircraft would be LOWER than indicated.


am I thinking correctly, but trying to understand a poorly structured sentence?!?


I could understand in a fixed volume that pressure would increase (Charles Law?) but we dont exactly have a fixed volume in the atmosphere.

Edited by Vassago
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I sketched this from my private level notes, it should make it a little easier to understand.


If you take off from a heliport with higher than standard pressure, and then fly to another with a lower than standard pressure, the altimeter will read the 26.00" pressure level at which you started cruising a thousand feet lower when you enter the lower pressure area if you do not change your altimeter setting. The mnemonic goes, "High to low, look out below, low to high, clear the sky."

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Pressure levels and pressure are two different things. The link Tom provides explains it well. This is the reason why many operators have cold temperature altimeter correction charts. Also why vnav approaches might not be authorized when it is to cold.

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