Definition
Atmospheric conditions at a given altitude that differ from the International Standard Atmosphere (ISA) values, which define standard sea level conditions as 59 °F (15 °C) and 29.92 inches of mercury (1013.2 hPa), with a temperature lapse rate of about 2 °C per 1,000 feet of altitude gain. Any deviation from these reference values, whether warmer, colder, higher, or lower in pressure, constitutes nonstandard temperature and pressure and affects density altitude, aircraft performance, and altimeter accuracy.
Plain English
The real air outside almost never matches the textbook 'standard' values for temperature and pressure. When it doesn't, the air is called nonstandard, and the aircraft will perform differently than the charts predict.
Context Anchor
Seen in weather, altimeter, and aircraft performance discussions, especially when comparing actual conditions to a standard day.
Derivation
Nonstandard' simply means 'not matching the agreed standard.' The standard here is the International Standard Atmosphere, a fixed reference adopted so that performance charts and instruments have a common baseline. Knowing the values are a reference, not reality, makes it clear why corrections are needed in actual flight.
Why Pilots Care
Nonstandard conditions cause altimeters to display incorrect altitudes and alter aircraft performance, requiring corrections to maintain safe separation and takeoff margins.
Grounding Statement
On a hot, low-pressure day, the airplane can act as though the airport is higher than it really is.
Intuition Check
Nonstandard does not mean illegal, unsafe, or strange. It means different from the standard reference conditions used for aviation calculations.
Example Sentence 1
On a hot summer afternoon, the pilot accounted for nonstandard temperature and pressure when calculating takeoff distance from the high-elevation airport.
Example Sentence 2
With nonstandard temperature and pressure present, the altimeter setting from the nearest airport was applied to correct the indicated altitude.