In aviation, the International Civil Aviation Organization (ICAO) created a standard for reference and performance computations that can be applied internationally. This is referred to as the ICAO Standard Atmosphere (ISA).
Under this umbrella term, atmospheric values including temperature, pressure, and density are used in relation to the altitude above mean sea level.
What is the Standard Temperature?
The standard temperature is 15 degrees celsius (50 Fahrenheit) as measured at a sea-level pressure of 29.92″ inches of mercury (Hg).
For every 1,000 feet that altitude increases, the standard lapse rate (the rate at which temperature or pressure changes with altitude) is 2 C (3.6F), up to the top of the stratosphere, which is 36,000 feet.
Above the stratosphere, the temperature remains constant up to about 80,000 feet. When the temperature and the standard lapse rate no longer match up, the term non-standard temperature then applies.
There is approximately a 1″ Hg decrease for every 1,000 feet that altitude increases too.
You should also be aware of temperature inversions. While it’s the norm for temperatures to drop as the altitude increases, when a layer of warmer air lies above a layer of cold air a temperature inversion occurs. In such an event, the temperature increases with altitude and can be spotted by moisture layers or trapped smoke.
How Does the Temperature Affect Aircraft Performance?
As you might expect, aircraft performance is evaluated within the standard atmosphere, meaning that all performance instrumentation is calibrated for these atmospheric conditions.
As operating conditions virtually never fit the standard atmosphere, corrections must therefore be applied to instrumentation and aircraft performance.
Standard Temperature vs. Pressure
As temperature and pressure are related and directly affect one another, it’s important to mention pressure too.
High pressure (when a mass air is more dense than the air around it) results in cooler temperatures and lower humidity.
Low pressure (when a mass of air is less dense than the air around it) results in warmer temperatures and higher humidity.
In regards to aircraft performance, at higher altitudes, pilots can expect engines to generate less power, wings to produce less lift, and propellers to generate less thrust.
Standard Temperature vs. Density
It’s also important to touch on the standard temperature and density.
Typically, air density varies inversely with temperature at a constant pressure. In other words, as the temperature decreases, density increases; as the temperature increases, density decreases.
However, in the atmosphere, temperature (and pressure) decreases with altitude, and the effect on density can vary. As a pilot, you can expect density to decrease with altitude.
How to Calculate ISA Standard Temperature
There is a pretty easy way to calculate ISA standard temperature for a given altitude. Simply double the altitude, subtract 15, and put a minus sign before it.
For example, to find the ISA temperature at 15,000 feet: double the altitude to 30,000 feet, subtract 15 to get 15, and place a minus sign in front to get -15.
If you want an exact calculation, you can always reach for your flight computer from your flight bag, but this ballpark will often be good enough.