surface air currents

I have a question regarding air currents above primary mirrors created by heating and/or cooling the primary. How much visual distortion will there be during warmup/cooldown and is it severe for very low power views? Will the effect be different depending on the aperture of the mirror (I already figure that the thickness will make a difference).

I'm assuming an open structure so that tube currents will be minimal. Thanks.

Best regards
Jess Tauber