What Causes Muggy Weather?

When Heat and Humidity Make Air Stiflingly Warm

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If you've ever endured a southern U.S. summer, the word muggy—a slang term used to describe unpleasantly warm and humid weather—is undoubtedly a part of your weather vocabulary.

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). 

 

Dew Point a Good Measure of Mugginess

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, dew point 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.

  1. Estimating mugginess using relative humidity can be misleading since a high relative humidity doesn't necessarily mean high mugginess. For example, on a 40°F day if the dew point is 36°F the relative humidity would be 90%. This is a high RH, but it wouldn't feel muggy because the air temperature is cool. In contrast, a 95°F day with a dew point of 67°F only gives a relative humidity of 70%, which is much less than our winter's day RH, but would feel a lot more humid!

    While not an official scale, the below will give you an idea of how muggy the air might feel at certain dew point ranges. As a general rule, if the dew point is 60 degrees or higher, the air will feel muggy.

    An Unofficial Mugginess Scale
    Dewpoint (°F)Degree of Mugginess
    < 50Not muggy
    50-59Slightly muggy
    60-69Moderately muggy
    70-79Very muggy
    79+Unbearably muggy

    (courtesy of Answers@NOAA.gov)

    High Dew Point + High Humidity

    The absolute worst combination for comfort is if both the dew point is high (65°F and above) and the relative humidity is high. When this happens, the air not only feels sticky and oppressive, but your body is at increased risk of heat illnesses, such as heat stroke and heat exhaustion!

    Sayings & Folklore

    Muggy weather is so uncomfortable, it often leads to many complaints, some of which have become traditional idioms, such as "The air is so thick, you could cut it with a knife!"

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    Your Citation
    Oblack, Rachelle. "What Causes Muggy Weather?" ThoughtCo, Mar. 22, 2018, thoughtco.com/muggy-weather-overview-3444058. Oblack, Rachelle. (2018, March 22). What Causes Muggy Weather? Retrieved from https://www.thoughtco.com/muggy-weather-overview-3444058 Oblack, Rachelle. "What Causes Muggy Weather?" ThoughtCo. https://www.thoughtco.com/muggy-weather-overview-3444058 (accessed April 19, 2018).