Standard Day

The term standard day is used throughout meteorology, aviation, and other sciences and disciplines as a way of defining certain properties of the atmosphere in a manner which allows those who use our atmosphere to effectively calculate and communicate its properties at any given time. For example, a temperature deviation of +8 °C means that the air at any given altitude is 8 °C (14 °F) warmer than what standard day conditions and the measurement altitude would predict, and would indicate a higher density altitude. These variations are extremely important to both meteorologists and aviators, as they strongly determine the different properties of the atmosphere.

For example, on a cool day, an airliner might have no problem safely departing a medium-altitude runway, but on a warmer day, the density altitude might require a higher true airspeed, which would require more acceleration, and more runway. The pilot may be forced to reduce fuel or cargo, or even add an intermediate fuel stop, delaying the flight arrival time. In meteorology, departure from standard day conditions is what gives rise to all weather phenomena, including thunderstorms, fronts, clouds, even the heating and cooling of our planet.