The Power of Evaporative Cooling

Step out of a pool on a warm summer day, or step out of the shower into a dry bathroom and you’ll experience firsthand the power of evaporative cooling.  As the water on your skin evaporates it takes heat from the air near your skin and the chilling affect can be dramatic.

Now imagine that process happening as billions of raindrops evaporate as they fall through a layer of the atmosphere thousands of feet thick, stretching across hundreds of square miles.  A lot of heat can be removed from the air, and that’s what happened Tuesday evening a few thousand feet above central Maryland, causing a burst of snow and sleet for much of the area.

The image above is the “sounding” of the atmosphere measured by a weather balloon launched around 7:00 p.m. near Dulles Airport in northern Virginia.  The red line traces the temperature as the balloon climbs and the green line traces the dew point.  The thick, slanted, blue line represents zero degrees celcius…the freezing line.  Everywhere the red line is plotted to the right of the freezing line, that layer of the atmosphere is above freezing.  The farther to the left the green line is plotted, the drier the air is at that point.  The Tuesday evening sounding clearly showed a pocket of warm, dry air a few thousand feet above the surface. Warm air at that level would almost always kill any snow chance, but the dry air changed equation. As rain fell into the warm, dry layer, evaporative cooling took place and the temperature cooled to around freezing, allowing a period of snow and sleet to develop. The wind barbs on the right side of the sounding show a southerly wind in the low levels of the atmosphere, which eventually re-warmed the snow producing layer, changing the wintry mix back to rain.

In a world of high tech doppler radar and sophisticated computer models, it is surprising for some to see that we still rely on good ole weather balloons and the data they collect. Unfortunately, these balloons are only launched a couple times each day and only from selected locations. The closest weather balloons for the Baltimore area are the ones launched near Dulles Airport, Wallops Island, VA, Pittsburgh, PA, and New York City…so there is some interpolation that needs to be done for those of us trying to nail down a forecast in central Maryland. Just one of the many challenging aspects to forecasting weather around here!

Tom Tasselmyer

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One Response

  1. wow! os so interesting! My daughter is studying that in sciense… she freaked out when she read it. Thanks for this!

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