Unless you are running an indoor track race, weather plays a critical role in running performance. A hot and humid day can have an especially negative impact on a race. The problem with high heat and humidity is that, together, they greatly reduce evaporation of sweat from the body. This prevents the body from dissipating the tremendous heat created by working muscles, which forces you to slow down.
So what is the ideal running weather in which to run a race? Well, simply asking about air temperature is not the right question. Rather, a runner should ask what the ideal “wet bulb temperature” is for a race. The “wet bulb temperature” measures how much water vapor the atmosphere can hold at current weather conditions. If the air is dry and can hold more water vapor, the wet bulb temperature is low – which makes it easy for sweat to evaporate. If the air is humid and unable to hold more water vapor, the wet bulb temperature is high – which makes it difficult for sweat to evaporate.
Wet bulb temperature is calculated by using the current humidity, air temperature, and pressure of your surroundings. Use the calculator here to calculate the wet bulb temperature (FYI, average “actual station pressure” at sea level is 29.92 inches of mercury).
A recent article discussed the the wet bulb temperature factor in races. According to the article, ideal racing conditions occur when the “wet bulb temperature” is 46° Fahrenheit or less. Using the calculator above, at sea level with 50% humidity, the ideal air temperature would be no more than 55° Fahrenheit to have the wet bulb temperature be less than the “ideal” temperature of 46° Fahrenheit. At sea level with 90% humidity, the ideal air temperature would be no more than 48° Fahrenheit.
Understanding how low the wet bulb temperature must be for you to run your best illustrates how difficult it is to run in hot weather with high humidity. For example, in this year’s Boston Marathon, the humidity at the start of the race was 66% and the air temperature was 66 Fahrenheit. Using air pressure at sea level, the wet bulb temperature was 57° Fahrenheit – way over the 46 degree ideal running weather number.
Keep the wet bulb temperature in mind when you run your next race in hot and humid weather. Don’t be demoralized by a slow time because it is simply difficult to run in such weather.
