I have the same problem when it rains and I have the A/C & Defroster on. The lower part of the windshield will fog on the outside of the windshield. If i turn off the defogger or A/C the fogging will stop and dissipate. The average temp here in Varina, Va during the summer is 86-95 degrees with 100% humidity. I would assume that the temp drop and the rain cooling off the windshield and the heat generated from the engine while the A/C or defogger are in use causes the fogging.