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February 16, 2010 / Dave Gorham

Sweat and Sleet: Connected

Sweat and sleet? Connected? Yes, but not the way you think… Sweating cools the body and sleet is the result of a cooling atmosphere. The common thread? Evaporative cooling.

Last week many areas of Texas and the Gulf Coast states experienced sleet — frozen or near-frozen droplets of water. The main question from nearly everyone I spoke with was: “But it’s only in the low 40s — how can that be?” It’s simple: Evaporative cooling.

 



Sweating cools the body. Photo: Nutra Legacy

You likely touched on evaporative cooling in high school, perhaps followed up a bit more in college. If that was a while ago, let me briefly refresh you: When water (or sweat) evaporates, it uses heat energy; as the heat is used the physical process of evaporation lowers the temperature of your skin.  The evaporation literally pulls the heat away from the surface of your skin.

The same principles apply to the atmosphere. Keep in mind, there are layers of the atmosphere where the air is colder and warmer than at ground level. If rain falls through a layer of much drier air before it reaches the ground (typically a few thousand feet above the ground) the rain droplets will begin to evaporate. At the same time, because evaporation uses heat, the droplets begin to cool. If that layer of air is dry enough, cold enough and thick enough, the liquid drops will freeze or begin to freeze as they fall through this layer. Closer to the ground, even if the temperatures are in the 40s, if the warmer air is not thick enough to melt the freezing/frozen rain, sleet will be the result.

 



A layer of sleet across a parking lot. Photo: Kidsgeo.com

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