Hot is bad. Humid is bad. But, Hot and humid is just plain nasty. Ever been on a run and it feels more like you're swimming than running? More than likely that's because the humidity is very high. High temperatures and high humidity is a rough combination on a runner.
Your body is an amazing machine. It has its own intricate cooling system. It's called sweat. The human body is all about maintaining balance. That's never more evident than with body's desire to keep a consistent internal body temperature. When it's hot outside and your internal body temp starts to rise from the heat and exertion, your body will begin to perspire. Perspiration is your body's method of cooling itself. When the sweat rises to the surface of your skin it evaporates. The evaporation of the sweat is what cools the body down. On a hot day, the body's cooling system works well. You run. You sweat. The sweat evaporates. Your body stays in balance.
If it's hot and humid, the body will produce the sweat, but because there's so much moisture in the air, evaporation happens much more slowly. That's why on a hot, humid day you appear to sweat more. You're actually not sweating more, but it appears so because the sweat has no where to go. It collects on your skin, your shirt, shorts, and if you're like me....down into your shoes (which at times will be so waterlogged, that they squish with each step).
Hydration is key no matter the conditions, but in hot/humid weather, the excessive fluid loss can really take a toll on your performance (not to mention your health). Some research shows that losing 2% of your body weight (through sweat) in your run can lead to a 4-6% decline in performance.
Heart rate also increases with increased heat/humidity. So your perceived effort is going to be greater when it's hot and humid. On top of this, all this sweating decreases your blood volume. That's because your body, in its effort to keep that balance, will divert more blood to the skin in an effort to keep it cool. That means less blood returns to the heart. That in turn means less oxygen-rich blood leaving the heart to fuel the muscles for your running. So, now your heart and lungs are working even harder to compensate trying to help you maintain your same pace as on a milder day.
A little trick that can help promote better evaporation of the perspiration is to pour some cool water over your head, neck and arms. The coolness will help promote better evaporation. It's not a cure-all, but it definitely helps. (Be sure to drink some of that cool water too!)
Sounds like maybe you shouldn't run in the heat and humidity, huh? Well, not so fast. Remember I said your body is an amazing machine? Training in the heat/humidity will provide some adaptation, but consistency is the key. But given that, training in the heat will only take you so far. Even with some acclimation, a lot of demand is put on the body in excessive heat/humidity.
So, this is where the attitude check comes into play. When it's excessively hot and humid, you have to adjust your expectations. If you're in training for a marathon and you have a speed workout such as 800 intervals, a tempo run, or a progression run, it's okay if you come in under your targeted training pace for that run.
Heat is an intensity, just like running a hill. If your normal tempo run is on a flat route and you change it one day to an extremely hilly route, you probably would end up with a slower overall time for that workout. But, you'd say, "Well, that was a hilly route." That same logic should be applied to a run on a hot/humid day. On hot/humid days pay more attention to perceived effort not pace. As long as you feel you put forth the same intensity on a hot/humid day as a cooler day, don't worry about what the clock says. You're still getting the same benefit training-wise.
Keeping all this in mind, there are days when the heat index is so severe that it may warrant skipping the run or dramatically altering your goal. In a Runner's World article, two-time Olympian Alan Culpepper presents a helpful guide for approaching your runs in various conditions. Culpepper says when checking the weather stats, look for the dew point instead of humidity. Dew point is the temperature at which water condenses. When the dew point is close to the air temp the more saturated the air is. This makes it harder for the sweat to evaporate off the skin for the desired cooling effect. So, dew point is a better tool in predicting how your performance is going to be affected.