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How to Calculate Atmospheric Transmittance

Transmittance is the measure of how much light gets from one place to another: from your headlights to a deer in the road, from an airborne observatory into space or from the top of one office building to another. The atmospheric transmittance is influenced by altitude, temperature, humidity and other factors. Calculating transmittance is easy in principle but difficult in practice.

Instructions

- 1
  Make an assumption about the molecular composition of the atmosphere. For example, you could assume that methane has a concentration of 740 parts per billion, that oxygen is at a concentration of 20.43 percent and so on through other atmospheric constituents.
- 2
  Find the absorption cross-section for each of the atmospheric constituents in your list. The cross-sections vary quite a bit with wavelength. Online databases will help you track this data down.
- 3
  Define the path through the atmosphere. The path will include the starting altitude, the distance and the viewing angle. That will lead to a density and temperature profile.
- 4
  Split the path into some different segments that have relatively constant density and temperature.
- 5
  Calculate the transmittance for each constituent in each segment. The transmittance is given by the exponential of the negative of the cross-section times the number density of the constituent times the distance. That is: transmittance = e ^-(cross section x number density x distance).
- 6
  Add all the contributions together and then add the segments together to get a plot of transmittance versus wavelength.

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