Standard day
Encyclopedia
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 thunderstorm
s, fronts, clouds, even the heating and cooling of our planet.
The first three properties are usually referred as "standard day" conditions, which the viscosity aspect is largely ignored throughout the aviation community. However, viscosity, which is affected by humidity levels, plays a key role in aerodynamic drag, which is why it is a key component of standard day conditions. Because it is a key component of drag, it affects the amount of fuel burned per unit of distance travelled.
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
, aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
, and other sciences and disciplines as a way of defining certain properties of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
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
Density altitude
Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...
. 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
True airspeed
True airspeed of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. True airspeed is important information for accurate navigation of an aircraft.-Performance:...
, 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 thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s, fronts, clouds, even the heating and cooling of our planet.
Standard day parameters
The "standard day" model of the atmosphere is defined at sea level, with certain present conditions such as temperature and pressure. But other factors, such as humidity, further alter the nature of the atmosphere, and are also defined under standard day conditions:- DensityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
(ρ): 1.229 kg/m³ - PressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
(p): 101.3 kPa - TemperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
(T): 15 °C - ViscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
(μ): 17.3 µPa·s
The first three properties are usually referred as "standard day" conditions, which the viscosity aspect is largely ignored throughout the aviation community. However, viscosity, which is affected by humidity levels, plays a key role in aerodynamic drag, which is why it is a key component of standard day conditions. Because it is a key component of drag, it affects the amount of fuel burned per unit of distance travelled.