When I was young, I used to get headache when I am exposed to the hot sun for a long time.
A magical cure that worked most of the time, is to put few drops of salted water at room temperature in my ears, for a minute or so.
Repeat this about 3 or 4 times for each ear alternatively. If you hear the water like boiling in your ear, and you feel it hot when it runs out, then this is a good sign that this will work.
Rest for about 10 minutes in a dark place. The headache sure will disappear.If any one knows why this work, please post the reason here.I think what happens is this, the blood is supposed to distribute heat evenly over the body, when it does not do this properly we get headache. Salt is good heat absorbant, and the ear drum is thin enough to let the salt absorb some of the heat and help the body get rid of it. Make sense?