Physical exercise is difficult in a humid atmosphere if you are not used to it. This is because sweat cannot evaporate into the air properly, making it very difficult for the body to cool down. Athletes and other sports players will train in humid conditions in order to prepare themselves for competition in such an environment.
The combination of warm weather and high humidity will slow you down and make your workouts much more difficult. However, high humidity levels aren’t an issue in cooler weather. While it’s difficult to determine the exact temperature that high humidity becomes a problem (due to all of the variables: body size, personal heat tolerance, etc.), most people need to pay attention when the air temperature hits 77° F (25° C), and the dew point is 57° F (13.9° C).
Think of “dew point” as the temperature at which dew drops form. When the dew point gets close to the temperature, the amount of moisture in the air becomes excessive. When you work out and perspire in these conditions, your sweat starts to fail at cooling you off.
The wetness of sweat is not what makes you cool. Cooling happens when your sweat evaporates off of your skin. Energy is required to turn liquid into gas. Your body heat is the energy source, so when your sweat evaporates, it takes your heat with it. When it’s hot and the humidity is high, the air is already saturated with moisture, causing this evaporation process to slow down or stop — leaving you drenched and uncomfortably hot.
Why is humidity not problematic in cool weather? Warm air has the capacity to hold a lot more water vapor than cool air. The hotter the air is, the more moisture it can hold. So even when the humidity is high on cooler days, it’s not an issue when exercising outdoors.
For example, I did an outdoor LTHR run in early January when the temperature was a chilly 38° F (3.3° C). According to Sport Tracks, the humidity was a steep 74% for that run, but it wasn’t a factor. I didn’t even start sweating until 10 minutes in, and when I did, it evaporated and successfully cooled me off. What made that run difficult was going as fast as I could for 30 minutes non-stop.
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