Definition
A pilot's memory aid warning that when flying from an area of higher temperature or higher pressure into an area of lower temperature or lower pressure without resetting the altimeter, the aircraft's true altitude will be lower than the altimeter indicates. The instrument continues to show the original altitude while the airplane is actually descending closer to the terrain below.
Plain English
If you fly into colder or lower-pressure air without updating your altimeter, you are lower than the altimeter says you are. The ground is closer than the gauge shows.
Context Anchor
Seen in altimeter error discussions, especially when cold weather affects terrain or obstacle clearance.
Derivation
This is a rhyme used as a memory aid. “Hot to cold” names the temperature change, and “look out below” warns that the airplane may actually be lower than indicated.
Why Pilots Care
Failure to apply the rule can result in controlled flight into terrain because the altimeter overreads true altitude in cold conditions.
Grounding Statement
Cold air makes the height between pressure levels smaller, so the altimeter can show an altitude that is higher than the airplane’s actual height.
Intuition Check
Do not assume the altimeter is showing your exact height above the ground in very cold air. In this phrase, “below” is the warning: your actual aircraft may be lower than the indicated altitude.
Example Sentence 1
Flying from the warm coast inland toward the cold mountains, the instructor reminded the student of the saying 'from hot to cold, look out below' and had her request a current altimeter setting before crossing the ridge.
Example Sentence 2
Crossing from warm valley air into colder mountain air, the crew climbed early after recalling the mnemonic to preserve terrain clearance.