Might goes lighter too, it has some red in it which comes out more when it's sunny. As others have said, it's the sunshine rather than the heat that does it and I get comments about whether I've lightened my hair. I'm the only one of my immediate family with lighter hair, everyone else has really dark hair.
I don't go out much in hot sun either, don't like it much and try to stay covered up in the shade but inevitable I have to just getting from work and back, at the bus stop, getting to shops etc...