My area of the country hit 90 degrees this week. As temperatures rise your need for fluids increases dramatically.
I found an article on the CDC.gov website recently that tells of a lawn care worker who died from heat stroke on a day when temperatures did not exceed 81 degrees. The fact there were contraindications from prescribed medications should not undermine the importance of proper hydration. This is a death that may have been avoided if the victim had been properly hydrated.
The main thing I learned from this article is that heat stroke can be caused by the cumulative effect of multiple days of improper hydration. If you don't hydrate properly one day and you don't catch up with fluid intake that evening you start the next day at a deficit. Cumulative effects give your body less ability to deal with heat stress over subsequent days.
Adhering to this article's recommendations may save your life this summer: