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The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a height above mean sea level .
at each geopotential altitude, where g is the standard acceleration of gravity, and R specific is the specific gas constant for dry air (287.0528J⋅kg −1 ⋅K −1). The solution is given by the barometric formula. Air density must be calculated in order to solve for the pressure, and is used in calculating dynamic pressure for moving vehicles.
A primary use of this model is to aid predictions of satellite orbital decay due to atmospheric drag. This model has also been used by astronomers to calculate the mass of air between telescopes and laser beams in order to assess the impact of laser guide stars on the non-lasing telescopes. [2]
Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmospheric pressure, temperature and humidity . At 101.325 kPa (abs) and 20 °C (68 °F), air has a density of approximately 1.204 kg/m 3 (0.0752 lb/cu ft), according to the International Standard Atmosphere (ISA).
"Density altitude is a measure of air density, expressed as the altitude that corresponds to a given air density under standard atmospheric conditions." As with air density, air pressure, temperature, humidity, etc., we can then talk about the density altitude observed at a given place and time, but places and times of observation aren't part ...
Comparison of the 1962 US Standard Atmosphere graph of geometric altitude against air density, pressure, the speed of sound and temperature with approximate altitudes of various objects. [ 1 ] The U.S. Standard Atmosphere is a static atmospheric model of how the pressure , temperature , density , and viscosity of the Earth's atmosphere change ...
Compressiblity is noticeable from Mach 0.3. And depends on what "noticeable" means. If by it we mean as commonly implied "noticeable on drag effects" then from 0.3 to up 0.7 the Prandtl–Glauert correction works decently. So you notice it but it's easy to calculate a "reasonably precise" value for most of the involved quantities.
Newton's experiments on drag were through air and fluids. He showed that drag on shot increases proportionately with the density of the air (or the fluid), cross sectional area, and the square of the speed. [9] Newton's experiments were only at low velocities to about 260 m/s (853 ft/s). [14] [15] [16]