It's the heat and the humidity

But cooling your body isn’t a complete solution. Dumping a cup of ice water over your head or putting one into your belly only provides momentary relief, and water-cooling jackets aren’t practical outside of physiology labs. So let’s look at what happens in real-world conditions. Our most efficient way of cooling off is sweating. When moisture evaporates from your skin, it takes some body heat with it.

In humid weather, though, sweat doesn’t evaporate as easily because the air is already full of water vapor. So when we’re talking about “heat,” we really mean something more like “perceived heat,” which is a combination of heat and humidity. This heat index chart shows the relationship: You’ll run slower in the heat (and humidity). While you can find charts like this one that predict how much slower you will run a race, the truth is that heat's effect on your running depends on whether you’re used to the heat, and on your body size. That’s right—not your fitness level, but your actual physical size.

People who are larger have more muscle, fat, or both. Muscle generates heat, and fat acts as an insulator. On the other hand, smaller folks generate less heat, but have more skin through which to dissipate that heat—the ol’ surface area to volume ratio. This is why petite runners place better in races on hot days. Some people think being more fit makes you better at dealing with heat, but that's only partially true: The fitter you are, the more body heat you produce, just because you’re so good at working hard.

Short of changing your body shape (which is possible, but hardly a short-term fix), what can you do to tolerate exercising in the heat better? The answer is simple: Spend more time exercising in the heat. Blue Lizard Baby Mineral Sunscreen Stick $8.