PDAS home > Contents > Standard Atmosphere > Non-Standard Atmospheres
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The standards group COESA has not defined the characteristics of a typical hot day or cold day, but there is a definition that has been released as MIL-STD-210A and MIL-STD-210C by the US Department of Defense. This MIL-STD defines a hot day, a cold day, a polar day and a tropical day. The definition extends from sea level to 30.5 km (100000 ft). In addition, it is common practice among some aerospace designers to define a non-standard day by an increment in temperature that is applied at all altituded to the standard day temperature. I have coded all of these atmospheres and the full source code is included with the CD-ROM (new in version 10, Jan 2005).

The computational algorithm for all non-standard atmospheres is as follows:

1. The standard atmosphere routine is used to compute temperature and pressure at the specified altitude. This pressure is the pressure in the non-standard atmosphere. The routine will also compute density, but it will be overwritten.
2. The temperature for the non-standard atmosphere is computed from its definition.
3. The density is computed from the perfect gas law.

The new subroutines in the package are:

• HotDay(altKm, sigma,delta,theta)
• ColdDay(altKm, sigma,delta,theta)
• PolarDay(altKm, sigma,delta,theta)
• TropicalDay(altKm, sigma,delta,theta)
The existing subroutines Atmosphere and SimpleAtmosphere have a new calling sequence:
• Atmosphere(altKm, sigma,delta,theta, deltaT)
• SimpleAtmosphere(altKm, sigma,delta,theta, deltaT)

The arguments are:

• altKm = geometric altitude in kilometers
• sigma = density/(sea-level standard density)
• delta = pressure/(sea-level standard pressure)
• theta = temperature/(sea-level standard temperature)
• deltaT = increment in temperature (kelvins) from a standard day

Some of you may be alarmed at this point, because you may have programs that use Atmosphere or SimpleAtmosphere that are coded with only four arguments. Not to worry! The new routines are coded with deltaT as an OPTIONAL variable. If you have
CALL Atmosphere(altKm, sigma,delta,theta)
you will get the same effect as if you had
CALL Atmosphere(altKm, sigma,delta,theta,0.0)
and you will not be flagged as being in error.

For additional information, I have a page that describes the MIL-STD-210 atmospheres and the extension to altitudes greater than 30.5 kilometers. A hot day is similar to a +15 K increment day and a cold day is somewhat similar to a -20 K day. However, the temperature profile for a cold day actually has temperature increasing from 0 to 1 km, then levelling off up to 4km, before beginning a decline as in the standard atmosphere.

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