Radiative heat is nothing at these temperatures, look at the formula: http://en.wikipedia.org/wiki/Thermal..._heat_transfer
Air flow above DEM at highway speed is turbulent, it will suck out every Joule out of DEM as long as DEMs temperature is below environmental temperature. Given that DEM's CPU prematurely fails due to overheat, and automotive grade electronics sustains heat at least of 85C (water boiling point minus 15C), air temperature should be really high to transfer heat in opposite direction.
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