Exploring the Relationship Between Optical Depth and Temperature/Pressure

[nordmoon]

What is optical depth? I have searched everywhere but I can't find any satisfying answer. It's stated often with radiative transfer that its the integral of the absorption coefficient for a certain distance. You usually distinguish between optical thin (tau <= 1) meaning you can 'see' through the medium or optical thick (tau >> 1) meaning you can't see through the medium. But what does it really say?

I have done some simulations of optical depth for a band of a certain species using data from the hitran database ( Tau = S(T)*f(v,vo,p)*u ) and I noticed that the optical depth (tau) decreases as you use higher temperatures and for low temperature you have a bigger tau values. In the same way but other way around, as you use higher pressure the broader the spectra becomes (I am guessing because of the lineshape dependence of pressure). What is the connection here?

 

[BigJDubs]

Optical depth is a measure of how much light from a source is absorbed or scattered as it passes through a medium, such as a gas or dust cloud. It is the integral of the absorption coefficient along a certain distance. The optical depth is a function of the temperature, pressure, and frequency of the light source. At low temperatures, the optical depth is large because the absorption coefficient is high. At high temperatures, the optical depth is small because the absorption coefficient is low. As the pressure increases, the optical depth increases because the lineshape broadens and more light is absorbed. This is why simulations of optical depth must account for these parameters in order to accurately predict the amount of light that will be absorbed.