Definition
An outside air temperature that differs from the temperature defined by the International Standard Atmosphere (ISA) for a given altitude. The ISA standard is 15 °C (59 °F) at sea level, decreasing by approximately 2 °C per 1,000 feet of altitude gain. Any actual temperature warmer or colder than this standard value at the corresponding altitude is nonstandard.
Plain English
The air outside the aircraft is warmer or colder than the textbook value used for performance charts and altimeter calculations.
Context Anchor
Seen in density altitude and aircraft performance discussions, especially when comparing actual weather to standard conditions.
Derivation
‘Nonstandard’ simply means ‘not matching the standard.’ The ‘standard’ here is the ISA — a fixed model of the atmosphere agreed internationally so pilots, engineers, and chart-makers all work from the same baseline. Knowing the baseline exists makes ‘nonstandard’ meaningful: it’s any deviation from that baseline.
Why Pilots Care
Nonstandard temperatures alter air density and therefore change density altitude, which directly affects takeoff distance, climb rate, and true airspeed.
Analogy
Think of standard temperature as the zero mark on a measuring scale. A nonstandard temperature is simply the actual temperature being above or below that reference mark.
Grounding Statement
On a hot day at a high airport, the airplane may accelerate and climb as if it were at an even higher altitude because the air is thinner than the standard model assumes.
Intuition Check
Nonstandard does not mean wrong, unsafe, or rare by itself. It means the actual temperature is above or below the aviation standard for that altitude.
Example Sentence 1
On a 35 °C summer afternoon at a 4,000-foot field, the nonstandard temperature pushed density altitude well above 7,000 feet, so the pilot recalculated takeoff distance before departing.
Example Sentence 2
Nonstandard temperatures required an adjustment to the published takeoff distance in the performance charts.