You already know that if you exercise outside on hot and humid days, you should drink plenty of water. And you are probably well aware of the risk of heat stroke given the countless reports about the warning signs.

But if you’re going to be out exercising anyway, you may have different questions: How long does it take to acclimate to the heat and humidity, and what is the best way to do it? How much does your performance time slow when it is sweltering and humid, and why? Does it help to douse your head with water?

Should you go out in the morning, when it is cooler but the relative humidity is higher, or at night, when it tends to be hotter but less humid?

The answers, some exercise physiologists say, are not always what you might expect.

There is no question that heat can take a toll on performance. Look, for example, at results from races in June 2008 when a heat wave gripped the Northeast.