Finally who has the same problems I do!!
This used to happen to me a lot when I lived in Africa. I was about 13 at the time. Whenever I was out in the sun I would get headaches.
It's because of these two things:
- acidity
- dehydration
Make sure you have a big breakfast in the morning before you leave for outside! If you don't have a lot of food in your stomach, you'll get acidity and you'll get a headache. Also, make sure you drink lots and lots of water. I'm talking about drinking a glass of water every hour or so! These two things seriously helped me out. I used to get headaches almost everyday for 3 or 4 months. I was a lazy bastard who refused to eat...eventually I smartened up.
btw, this also happens over here in Canada sometimes too (in the summer). Also, another simpler cause would be heatstroke..but I don't think there is much you can do about that then stay in the shade.
btw, just remembered something..do you have sensitive eyes? wear sunglasses when your in the sun, that may help since do wouldn't strain your eyes (or squint) so much.
Hope this helps...I have no medical background (I'm only 18), but I suffered with headaches quite a bit.