Mon June 3, 2013
Dr. Dale Durran, University of Washington – Humidity and Cold Drinks
In today’s Academic Minute, Dr. Dale Durran of the University of Washington explains why it’s more difficult to keep your drink cold on a humid day.
Dale Durran is a professor of atmospheric sciences and an adjunct professor of applied mathematics at the University of Washington. His research interests include mesoscale dynamic meteorology, numerical methods, mountain meteorology, and atmospheric waves. His work has been published in a number of peer-reviewed journals and he holds a Ph.D. from the Massachusetts Institute of Technology.
Dr. Dale Durran – Humidity and Cold Drinks
In spring our skies are filled with puffy cumulus clouds, whose turrets grow upward because, like hot air balloons, they are warmer and less dense than the surrounding air. But, unlike hot-air balloons, the warmth inside cumulus clouds is not produced by burning propane; instead the heating inside a rising turret is produced by water vapor condensing to form cloud droplets.
We experience the reverse process, evaporative cooling, when our bodies give up the heat required to evaporate sweat from our skin. But we have very little intuition about the power of condensational heating. To improve your intuition, and to better understand cumulus clouds, consider a 12-oz can of cold beer. Suppose the can has become covered with a uniform layer of condensation having the thickness of a human hair. If all the heat released by this condensation is transferred to the beer, how much does the beer warm? A straightforward theoretical calculation gives the answer: nine degrees Fahrenheit!
How does this actually apply to your canned beverage? To find out, Professor Dargan Frierson and I measured the rate at which aluminum cans filled with 12 ounces of ice-cold water warmed inside an environmental chamber as we varied the air temperature and the humidity. It turns out, the rate of condensational heating is indeed significant on hot humid days. On a day when the temperature is
87 degrees Fahrenheit and the relative humidity 70%, the condensation that forms over five minutes will heat the can’s contents by 10 degrees, and this condensational heating will exceed the dry heat transfer from the surrounding air. World-record humidities are observed near the Persian Gulf, and under those conditions, the five-minute condensational heating increases to 16 degrees. When your beer is warming on a hot muggy day, it’s not just the heat, its the humidity.