Definition
Outside air temperatures that are notably lower than the standard temperature for a given altitude, cold enough to cause the aircraft's true altitude to be lower than the altitude indicated on the altimeter. In icing-related contexts, the phrase also flags conditions where ice accumulation behavior, fuel system performance, and instrument response can differ meaningfully from temperate-weather expectations.
Plain English
Air that is much colder than normal for the altitude you're flying at. When this happens, your altimeter can read higher than you actually are, and cold-weather effects on the airplane become more pronounced.
Context Anchor
Seen in icing discussions when a pilot is deciding whether climbing, descending, or changing course will move the aircraft into safer or worse conditions.
Derivation
“Significant” comes from a root meaning “to be a sign of something.” In this phrase, it means the colder temperature is a meaningful sign for the pilot, not just a small or casual change.
Why Pilots Care
These temperatures change how ice builds on wings and surfaces, often requiring earlier or more aggressive use of anti-ice and de-ice systems.
Grounding Statement
A small temperature change may not matter, but a large drop can mean the aircraft is entering a different and more serious weather situation.
Intuition Check
Do not read “significantly” as a fixed number of degrees. Here it means “large enough to affect the flight decision.”
Example Sentence 1
When operating into a mountain airport during winter, significantly colder temperatures required the crew to apply a cold temperature altitude correction on the approach.
Example Sentence 2
The briefing noted significantly colder temperatures along the arrival, prompting an earlier descent to stay out of the icing layer.