Mirror seeing is caused by natural or weakly mixed convection over a mirror warmer than ambient air. The seeing effect is generated in a thin region just above the viscous-conductive layer where the temperature fluctuations are largest and most intermittent. If its cause could be visualized, seeing would appear to come from a thin but very turbulent layer "floating" a few millimeters above the surface.
On the base of this physical description we have expressed the hypothesis that the average amplitude of the seeing would be essentially a function of the surface heat flux. The profile of can then be described by a similarity equation (), valid down to the interface with the tiny viscous-conductive layer. Computations of mirror seeing with this similarity model show a good agreement with experimental results.
The results of different experiments, done by various researchers, have been processed to get a homogeneous database. In absence of forced air flow, mirror seeing appears not to depend on the mirror size. The effect of inclination is more controversial: it is reported to be large for a small 25-cm mirror but does not appear in our analysis of the 3.6-m CFHT telescope. For the purpose of engineering parametric studies the following relationship is proposed:
with a possible spread of 25%.
In the case of a ventilated mirror, the seeing decreases showing a dependency on the Froude number. The relationship derived from the experiments is:
The dependency on the Froude number suggests the influence of the mirror size. Thus larger mirrors should produce more seeing in the same conditions of ventilation.