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?