I don't have a heat joke per se,but what I'm about to post gives new meaning to "It's so hot,you could fry an egg on the sidewalk"
I just moved to Texas. Anyhoo,on one of the news programs, the news lady want to show how hot it can get. So she took a cookie sheet and a package of cookie dough. She put the dough on the cookie sheet and put the cookie sheet on the dash board of a truck and 30 minutes latter she had freshed baked cookies.