Cramps will not only affect the target muscle groups, but they can also hit your stomach, causing gastrointestinal distress. Performance will continue to be compromised as you dry out. Headaches, disorientation, and irritability follow. None of those things will make you faster.
Many athletes remark that they sometimes don't urinate for hours after a race. This is a sign that they are under-hydrating. When you pee, it should be lemonade-colored, not apple juice.
Humidity will also impact your hydration levels. Remember the example about arriving in Hawaii or Florida? The humidity is sucking the liquid right out of your skin, and that needs replenishing.
The flip side to the humidity issue is going to a very dry environment. Simply inhaling requires fluid, so even when you aren't sweating, you should still be aware of the water exchange occurring inside your body.
While sleeping in the brutal California desert, your body processes about a pint of water an hour. The environment is so dry that every breath goes in dry but comes out wet. So after eight hours of sleep, you will have used a gallon of water. A dry climate is just as serious as a humid one.
It should be mentioned that over-hydration is also a risk. Hyponatremia (low blood sodium) occurs when an athlete over-dilutes the sodium in their bloodstream. Salt tablets and electrolyte-heavy sports drinks help prevent this. Keep in mind that while hyponatremia is real, it is much less of a risk in hot weather than dehydration.
Hydration should not only happen during an event. Pre-race hydration is equally important. While you do not want to be sloshing your way toward to start line, you should be drinking a slightly higher than normal volume of water for a few days prior to your race.
Acclimation, general fitness, and hydration are the best ways to prepare for and complete a hot weather event. If you feel sick, stop to find water and shade. Be safe out there. Finishing slow is better than not finishing at all.Sign up for your next race.