Definition
A pilot's memory aid warning that the altimeter will read higher than the aircraft's actual height above terrain when flying from an area of warmer air into colder air, or from an area of higher atmospheric pressure into lower pressure, without resetting the altimeter. In both cases, the true altitude is lower than indicated, so the aircraft is closer to the ground than the instrument suggests.
Plain English
If you fly into colder air or into an area where the pressure is lower, your altimeter will lie to you by reading too high. You're actually lower than it shows, so terrain and obstacles are closer than you think.
Context Anchor
Seen in altimeter error discussions, instrument flying, cold-weather operations, and route planning where terrain clearance matters.
Why Pilots Care
Failure to correct for this error can place the aircraft below a published minimum altitude or into terrain while the altimeter still shows a safe reading.
Grounding Statement
In colder or lower-pressure air, the altimeter overreads, so the aircraft is lower than it shows.
Intuition Check
Do not read “high to low” as high altitude to low altitude. In this phrase, it means high pressure to low pressure. “Look out below” means the danger is being lower than the altimeter appears to show.
Example Sentence 1
Flying from Phoenix in the summer heat into a cold front over the mountains, the instructor reminded the student: from hot to cold, look out below.
Example Sentence 2
During the approach briefing the instructor reminded the student: from a high to a low, look out below.