5.2 Atmospheric Condition and Altitude

Atmospheric condition is different depending on the altitude. This factor must be considered in the selection of platforms or sensors. In this section, air pressure, air density and temperature are considered.

Dependence of air pressure on altitude is based on the hydro-static equilibrium of balance between the vertical pressure of the atmosphere and gravity.

The atmospheric constituents without water vapor are assumed constant in volume ratio with 78.08 % nitrogen, 20.95 % oxygen, and argon 0.93 % up to about 100 km regardless of time and place. It gives an average molecular weight at 28.97 for the atmosphere and the average molecular mass of 4.810 x 10 kg.

When temperature is constant with respect to altitude, the air pressure decreases as an exponential function, which gives about an 8 km altitude for a decrease of air pressure to 1/e, as shown in Figure 5.2.1.

However, since the actual atmosphere varies in temperature with altitude as shown in Figure 5.2.2, the air pressure can be calculated from the hydro-static equilibrium with a given temperature.

For general purposes, the standard model atmosphere has been specified with respect to the average temperature distribution and the vertical air pressure. Also the average model with respect to latitude and season has been specified, although the actual temperature sometimes has a difference of 10 - 20K. Therefore the measurement of temperature using radio-sonde is necessary for high accuracy. The vertical structure of the atmosphere is composed of the following layers.

Troposphere : from the ground surface to 10 - 17 km,
Stratosphere : from 10 - 17 km to about 50 km
Mesosphere : from about 50 km to about 90 km
Thermosphere : from about 90 km to 500 km

The classification of the above layers depends on the distribution of thermal energy and thermal transportation. The vertical decrease of temperature in the troposphere is 9.8K km for dry atmosphere, but 6.5K km for the actual atmosphere because of water vapor. The boarder between the troposphere and the stratosphere is called tropopause. The tropics tropopause is rather constant at 17 km in altitude while the middle latitude tropopause depends on seasonal change and jet stream with 10 - 17 km in altitude as shown in Figure 5.2.3.


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