I believe heat can be a major trigger for migraine headaches, as some of the worst headaches I've ever had have come after prolonged exposure to heat.  The worst headache I've ever experienced was last summer.  My husband and I had been outside mulching a house that we were in the process of selling.  The temperature was in the mid 90s, and we were outside for at least 2 hours.  We both drank plenty of water.  Towards the end of the job, I began to feel like I wasn't able to function.  Everything seemed to slow down.  I sat in the shade until my husband finished.  As soon as we sat in his truck, I felt as though someone had hit me in the head with a 2 x 4.  He took me home immediately and I took a Maxalt that had been prescribed.  It didn't help and the headache and vomiting became worse, so I took another.  I got so bad that I couldn't see, I couldn't stand, I could barely function.  As if that wasn't bad enough, when the headache and vomiting finally subsided, I continued to feel weak for at least 3 or 4 days