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Weather Talk: Hot, humid air is light, not heavy

Perceptions can be misleading. An example of this is the fact that humid air is light compared to air with low humidity.

One common expression, "It's so humid you can cut the air with a knife," illustrates the feeling that humid air is oppressive and, therefore, heavy. Actually, air that is more humid is lighter than air with low humidity.

The explanation is fairly simple: Air is about 78 percent nitrogen and about 20 percent oxygen. Water vapor accounts for about 1 percent on average, but the amounts vary from place to place as humidity goes up and down.

It turns out that each molecule of water vapor is lighter than each molecule of either nitrogen or oxygen. So the more water vapor (humidity) there is in the air around you, the lighter the air is.

If you feel sluggish in sticky weather, it isn't because of the weight of the air.

John Wheeler

John was born in Baton Rouge, LA, and grew up near Birmingham, Alabama. As a teenager, his family moved to Madison, Wisconsin, and later to a small town in northeast Iowa. John traces his early interest in weather to the difference in climate between Alabama and Wisconsin. He is a graduate of Iowa State University with a degree in meteorology. Like any meteorologist, John is intrigued by extremes of weather, especially arctic air outbreaks and winter storms.  John has been known to say he prefers his summers to be hot but in winter, he prefers the cold.  When away from work, John enjoys long-distance running and reading.  John has been a meteorologist at WDAY since May of 1985.

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