I had an interview one June when it was over 100. I went in wearing a suit and tie, and then discovered that the AC in the building had gone on the fritz. The principal shook my hand and said, "Okay, you've impressed me. Take off your tie and jacket...and here's a bottle of water."

If I had it to do over again, I probably would still go suit & tie. With that said, depending on what you're wearing, there are times that going tieless looks just as good.