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In a bit more detail.....
When the mirror is significantly hotter than the ambient air temperature there is a steep thermal gradient formed and a heat transfer takes place from the mirror to the surrounding air. A large gradient can be disasterous causing all sorts of convective turbulences to the boundary layer of air that sits above the mirror surface. The more turbulent this is, the less performance you get out of your scope. Even at mid powers stars are boiling blobs of light instead of the normal pinpoints due to the light scatter (interference) caused by the boundary layer. What Ken said is also true - the figure of the mirror can be distorted due to temperature, but it's contribution to poor performance is not as great as the boundary layer above the mirror surface.
Time will heal this eventually (get your scope out as early as possible), unless the gradient is so steep that it just can't keep up with the ever changing ambient temp. Keeping the mirror above ambient will also keep the dew away to some extent, and ideally you would want your mirror to be no more than a degree or two warmer than the surrounding air.
My dob has fans that blow across the mirror, although I find most of the time I don't need them. Their operation is currently pretty temporary, but once I make the connections more permenant I will probably use it more. I plan on testing this out more (observing with fans on, etc) in the coming weeks and if I find anything of value I will start a thread on it.
There are heaps of discussions going on around the web in regards to where to place the fans and whether to have them sucking or blowing and I think the jury are still out on those debates (do what ever works for you!) as there is not enough evidence either way. There have been some articles in S&T in recent years regarding mirror cooling (I think by Bryan Greer? sp?) and thermodymanics in a close tube situation that are good reading.
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