We have already mentioned that the seeing is due to turbulent fluctuations of the index of refraction. The index of refraction N varies with the density and composition of the medium. In air it may be expressed by Cauchy's formula as a function of wavelength , pressure P (mb), absolute temperature T (K) and water vapor pressure v (mb):
Fluctuation of humidity are only significant in extreme cases like fog or near the sea surface which are not relevant to astronomical observations. Since also pressure effects are negligible, turbulent fluctuations of the index of refraction are practically only due to temperature fluctuations and therefore intimately linked to the structure of thermal turbulence in the atmosphere.