Plain English Meaning
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.
Full 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.
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.