Optical Turbulence
This webdemo simulates the effect of atmospheric turbulence on free space optical(FSO-)signal

Turbulence occurs because of the random fluctuation of the refractive index $\mathrm{n}$ due to variations in temperature and pressure.
Indeed, the refractive index $\space\mathrm{n}$ of the air for visible wavelengths is a function of the temperature $\mathrm{T}$, the pressure $\mathrm{P}$ and the wavelength $\lambda$ according to the following equation[1] \begin{align} \mathrm{n(r)} = 1 + 7.66 \cdot 10^{-6} (1+7.52\cdot 10^{-3} \cdot \lambda^{-2}) \left(\frac{\mathrm{P(r)}}{\mathrm{T(r)}}\right) ≈ 1+79 \cdot 10^{-6} \left(\frac{\mathrm{P(r)}}{\mathrm{T(r)}}\right) \end{align} For the refractive index fluctuation, the temperature variation is more important than the variation of pressure and $\lambda$ .
According to the above equation we can thus write for the refractive index fluctuation: \begin{align} \partial \mathrm{n}\space = \space-79 \cdot 10^{-6} \left(\frac{\mathrm{P}}{\mathrm{T}^{2}}\right) \partial \mathrm{T} \end{align} \begin{align} Let   \mathrm{C_{n}} = \frac{\partial \mathrm{n}}{\partial \mathrm{T}}\space\mathrm{C_{T}} = -79 \cdot 10^{-6}\left(\frac{\mathrm{P}}{\mathrm{T}^{2}}\right) \mathrm{C_{T}} \implies \mathrm{C^{2}_{n}}= \left(79 \cdot 10^{-6} \left(\frac{\mathrm{P}}{\mathrm{T}^{2}}\right)\right)^{2} \mathrm{C^{2}_{T}} \end{align} $\mathrm{C^{2}_{T}}\Bigl[\frac{\mathrm{deg}^2}{\mathrm{m}^{2/3}}\Bigr]$ is the temperature structure constant. It is a measure of the temperature fluctuation between 2 points separated by a certain distance along the propagation path[1]

$\mathrm{C^{2}_{n}}\bigl[\mathrm{m}^{-\frac{2}{3}}\bigr]$ is called refractive index structure constant and is a measure of the strength of fluctuations in the refractive index[1] $\mathrm{C^{2}_{n}}$ ranges generaly between $10^{-17}$ or less (in case of weak turbulence) and $10^{-13}$ or more (in case of strong turbulence). The turbulence is then moderate for $\mathrm{C^2_n}$ between $10^{-17}$ and $10^{-13}$