You could do a double render to get the heat ripples a distance render so you can pick out hte right parts and a regular render. To get a better abmient light I use a lighting rig of around 64 lights in a sphere configuration over the scene with very low level and no falloff.  Old tricks.

The wit of a misplaced ex-patriot.
I cheated on my metaphysics exam by looking into the soul of the person next to me.