I'm pretty sure it is because of the weather too. My vented hood lets out excess heat and when going down a long hill, I'll let off the gas and just coast so I can watch my gauge get near C. And I live in S. Cali. and it will do this at almost any part of the day, depending on how steep the hill is. I wouldn't worry about it. Just be glad it's not going the other way. I've never heard of over cooling causing any damage.