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Originally Posted by bird
2. If the mirror is warmer than the ambient air then a layer of unstable air forms over the mirror (called a boundary layer) which distorts the image very badly, as light refracts differently through air at different temperatures.
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Think of the heat haze effect coming off a hot road in summer. The principle is the same with a mirror that is not at the same temperature as ambient. To get maximum image sharpness and stability, we must get rid of this boundary layer.