So how does this work? For fertilization to take place, a pollen grain must land on the stigma of the flower. A pollen tube is then formed which grows from the stigma, down through the style to the ovule where fertilization take place. Excessive heat literally causes the pollen tube to explode, thereby preventing pollination. Even short bursts of temperatures above 95 degrees are enough to cause this reaction. Once temperatures cool, pollination can again take place.
I mentioned before that there is some variation among varieties in how susceptible they are to heat preventing fruit formation. There are "heat-set" slicing tomatoes such as Florida 91, Sun Leaper and Sun Master that will set fruit at higher temperatures, but that difference is normally only 2 to 3 degrees. (Ward Upham)