Relative Humidity (RH) is not a good value to use when one is considering sporting activities such as running. RH only tells us how close the air is to being saturated with water vapor so RH is quite misleading. On the other hand, Dewpoint Temperature (TD) is directly related to the actual amount of water in the air so it is the best value to consider. For the warmer season when we are outdoors a TD value in the 60-64 range is considered “normal”, 65-69 is “humid”, while 70-74 is “muggy”. TD = 75+ is just plain obnoxious! Values below 60 are considered to be dry air.
RH is a misleading value because it is influenced by the actual amount of water in the air as well as the current air T. Although the physics of the following analogy is oversimplified, one can think of the air as a sponge. Higher air T means the air can “hold” more water much like a larger sponge holds more water. Cooler air would be like a much smaller sponge. RH merely measures how close the sponge is to being filled. A much smaller sponge can have an RH of 100% but still contain little water. On the other hand, 50% RH in a very large sponge (hot day) would contain much more water. That day would have a high TD even with the low RH and would feel miserable.
Today we have TD values near 75 so that is why it is so muggy. Unfortunately, as humans keep dumping massive amounts of heat-trapping gases such as carbon dioxide into the air, the planet is becoming warmer. Because the planet is 70% ocean, this warmer water is evaporating into the air more rapidly, not only causing our sponge to be larger but also to contain even more water. Expect these muggy days to increase over the coming years if we do not rapidly begin shifting to less carbon-intensive energy sources.
