What Makes It Muggy?

Like the heat index, muggy is a "feels-like" condition, except it has to do more with how "breathable" the air feels than how hot it feels. The muggier the weather, the less chance you'll feel cool because of decreased evaporation rates, which is why the following weather conditions are notoriously linked to the muggiest of days and nights:

Warm air temperatures, generally of 70°F or above (the warmer the air, the more moisture it's able to hold);

High moisture (the more moisture there is in the air, the "heavier" it feels); and Low winds (the less wind there is, the fewer air molecules there are passing over your skin evaporating and cooling you off).

Since mugginess expresses how moist the air feels, you might think that relative humidity would be a good indicator of how muggy it feels outside. However, dewpoint temperature is actually a better measure of mugginess. Why? Dewpoint not only gives you an indication of how moist air is, but how warm it is as well (since dew point temperature can go as high as, but never higher than the actual air temperature). So if the dew point is high, it means both air moisture and temperature probably are, too.