What makes it feel “muggy”?

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What makes it feel “muggy”?

Summer days are referred to as “muggy” when they are very warm and humid. The word “muggy” is believed to come from the Middle English “mugen” which means drizzle. It may also arise from the fact that on such a day, a cold mug becomes covered with a film of liquid water when exposed to humid air.

Air’s humidity is directly related to the amount of water vapor it holds, which can vary considerably. During winter, the water vapor content is very low, as cold air has a quite limited capacity to hold water vapor. Warm air, however, can hold considerable amounts of water vapor. In fact, on July 13, 1995, each breath of air in Madison was about five percent water vapor!

High water-vapor content contributes to respiratory distress in some people and so extreme warmth and humidity can be a serious health risk. High water vapor content is also a key fuel for severe thunderstorms.

Where does the water vapor come from? Southern Wisconsin has two major sources; the Gulf of Mexico and Iowa! As strange that sounds, maturing corn plants are a huge source of water vapor to the atmosphere. In fact, so much water vapor is evapotranspirated (released by the plants during respiration) from Iowa corn in July that the entire state acts like a shallow sea!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.