Definition
A pilot's memory aid warning that when flying from an area of high atmospheric pressure into an area of lower atmospheric pressure without resetting the altimeter, the altimeter will indicate an altitude higher than the aircraft's actual altitude above sea level. The aircraft is therefore lower than the instrument shows.
Plain English
If you fly into air where the pressure is lower and you don't update your altimeter, the altimeter lies to you by reading too high. You're actually closer to the ground than the gauge says.
Context Anchor
Used when learning altimeter errors caused by changing pressure, especially during cross-country flight planning and in-flight weather changes.
Derivation
This is a pilot memory phrase, not a word origin. “High to low” means moving from higher pressure to lower pressure, and “look out below” warns that the airplane is lower than the altimeter may show.
Why Pilots Care
Failure to account for this pressure change can result in the aircraft being hundreds of feet lower than the pilot believes, increasing the risk of controlled flight into terrain.
Grounding Statement
As outside air pressure drops, an altimeter left on the old higher setting will read too high.
Intuition Check
This does not mean flying from a high altitude to a low altitude. It means flying from high air pressure to low air pressure, where the altimeter can make the aircraft appear higher than it really is.
Example Sentence 1
As we crossed into the low pressure system without updating the altimeter setting, I reminded myself: going from a high to a low, look out below.
Example Sentence 2
During cross-country flight planning the instructor emphasized going from a high to a low, look out below, to keep the student alert for unadjusted altimeter errors.