Many things can affect running performance and enjoyment, including nutrition, hydration, rest and conditioning. If you live in a southern climate, you're probably very familiar with another major factor: humidity. When people speak of humidity, which is moisture in the air, they're usually referring to "relative humidity." This term refers to the amount of water vapor in the air as a percentage of the amount the air could hold at a specific temperature. Heat and humidity are often expressed in conjunction, but humidity can affect running even if it isn't extremely hot out.
Effects of Humidity on the Body
Humidity can effect your core temperature because it disrupts your body's natural cooling system. When you sweat, the process of evaporation reduces body heat. However, when the air is very humid, evaporation slows, reducing your body's ability to cool off. The excessive heat from hindered cooling causes the body to direct more blood to the skin to transport heat out of the body. As a result, there is less blood for muscles to utilize, which reduces running performance. Heat also causes the body to become glycogen depleted more quickly, emptying the stores of energy that muscles need to work.
Humidity and Running
When your body becomes excessively hot during running, you will be fatigued and your pace will likely suffer. It's important to adjust training intensity and race expectations based on relative humidity. Above 65 degrees Fahrenheit, your heart rate will become elevated by an extra 10 beats per minute. In humid conditions, it can rise an additional 10 beats. Remember that humidity can cause temperatures to feel much hotter than they really are -- for example, at 80 percent relative humidity, 85 degrees Fahrenheit can feel like 97 degrees Fahrenheit. This extra heat causes a struggle between your body's muscular and cooling systems, both battling for blood. Meanwhile, excessive sweating can reduce blood volume, severely hindering both systems.
To combat performance and health issues related to humidity, you must give your body all the tools you can to help it stay cool. Wear light-colored, lightweight clothing that wicks sweat from your body and encourages sweat to evaporate from your skin. Train during times of the day that are most comfortable. The air may contain more humidity in the morning hours, but if the afternoons heat up considerably, it may be better to train in high humidity and cool temperatures than in moderate humidity and high temperatures. Adequate hydration is the singly most important key to running in humid temperatures. You must account for extra fluids lost from sweat, and thirst is not an accurate indicator of hydration. Drink 4 to 8 oz. of water or sports drink for every 15 to 20 minutes spent running.
Be aware of the signs of heat cramps and heat exhaustion, which include muscle cramps, profuse sweating, nausea and lightheadedness. Stop and rest in a cool place if you become overheated. If you stop sweating or begin to hallucinate, this is an indicator of heat stroke, which should be immediately treated as a medical emergency. Give your body time to acclimate to humid conditions. If you're accustomed to training in dry air, running in humidity can be a demoralizing experience at first. Allow at least two weeks to adjust to the new conditions.
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