Atmospheric Modeling

Obviously, the local atmospheric conditions affect the airplane. Air density and the wind speed are the most important conditions. Other conditions, such as pressure and temperature, affect some of the instruments.

The Standard Atmosphere model $^{\ref{ref:stdatm}}$ gives the average temperature, pressure, and density (over time) at any point in the atmosphere, as a function of altitude. For altitudes less than 36000 ft, the average temperature as a function of altitude (which is an empirical formula) is:

$$\overline T = \ensuremath{518.69^\circ}\textrm{R} - (0.003566\,^\circ\textrm{R/ft})h$$ (2)

(Reference 4 lists relations valid for higher altitudes.) The average pressure (which derives from the hydrostatic equation and the ideal gas law) is:

$$\overline p = (2116.8~\textrm{psf}) \left(\frac{\overline T}{\ensuremath{518.69^\circ}\textrm{R}}\right)^{5.262}$$ (3)

Then, the ideal gas law gives the average density:

$$\overline\rho = \frac {\overline p}{R \overline T}$$ (4)

Equations 2-4 only give the average conditions at a point in the atmosphere, hence the overbars. Depending on the weather, the temperature and pressure could be much higher or lower than the average. Many flight simulators have meteorological models to simulate these fluctuations.

Unlike density, there is no standard model for wind; it is (almost) entirely dependent on the weather. Again, a meteorological model can be used. In the absence of a meteorological model, the user could simply input a velocity vector for the wind he or she wants to fly in (this could be useful to practice landing in a crosswind, for instance).

Some types of wind are easily predictable and not much dependent on the weather. For example, thermal updrafts and downdrafts occur over specific locations at specific times of day.