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On Earth, air is a gas comprised of the following: 78% nitrogen, 21% oxygen, and trace amounts of carbon dioxide, water vapor, and other things.

At sea level, standard values for air characteristics are:

• Density (r = mass / volume): 1.229 kg/m³; 0.00237 slug/ft²
• Specific Volume (V = volume / mass): 0.814 m³/kg; 422 ft³
• Pressure(p = Force / area): 101.3 kN/m²; 14.7 lb/in²
• Temperature (T = degrees): 15 Celsius; 59 Fahrenheit
• Viscosity(mu = force-time / area): 1.73 x 10^(^-5) N-s/m²; 3.62 x 10^(^-7) lb-s/ft²

If you are interested, NASA has created an interactive applet for atmosphere research on Earth (based on altitude): http://www.grc.nasa.gov/WWW/K-12/airplane/atmosi.html

Earth's atmosphere is divided into several levels at which temperature (T in degrees Fahrenheit) and pressure (p in pounds/cubic feet) vary according to height (h in feet). The graphic at left shows these different levels.

In general, as altitude increases, density, pressure, and temperature decrease. Density (r in slugs/cubic feet) is directly related to pressure and inversely related to temperature via the following equation:

r = p / [1718 * (T + 459.7)].

Troposphere (height less than 36,152 feet)

T=-205.05 + 0.00164*h
p=51.97*[(T+459.7)/389.98]^-11.388

Lower Stratosphere (height between Upper Stratosphere and Troposphere)

T=-70
p=473.1*e^{73-0.000048*h}

Upper Stratosphere (height above 82,345 feet)

T=59-0.00356h
p=2116*[(T+459.7)/518.6]^5.256

These values are further explained at: http://www.grc.nasa.gov/www/K-12/airplane/atmos.html

In deep space, temperatures range from absolute zero (in shadows) to very high values when exposed continually to ultraviolet radiation. Density is approximately zero. This influences aerodynamics tremendously, as density is an important variable related to gas behavior. Most influential of all, however, is the absence of gases in the vacuum of space. Propulsion systems that use air, such as propellers or wings, are of no use in space.

The Earth's magnetic field extends out into Space, over a radius (from the Earth's core) of 36,000 miles.

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