Turns out there's a scientific justification for beer koozies (those foam-rubber beverage sheaths the local radio station gives away at the county fair): They shield your drink against the droplets of water that form on the outside, which can put the heat on your drink.
“Probably the most important thing a beer koozie does is not simply insulate the can but keep condensation from forming on the outside of it,” University of Washington atmospheric sciences professor Dale Durran said in a statement.
Turning water into water vapor requires heat -- think of boiling water in a teapot. When the water vapor condenses again into a droplet, it loses that heat, which then warms the surrounding air.
Durran and his colleague Dargan Frierson were interested in finding a good way to demonstrate the power of condensation heating, and they hit upon the idea of using cold beverages. They've determined that droplets of water forming on a can or bottle can provide way more heat than the surrounding air – so the more humid it is, the harder it'll be to keep your soda cold. A blazing hot day in the dry heat of Arizona might not be as bad for your ice-cold drink as a balmy but sticky day in New Orleans.
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In an experiment recently published in Physics Today, Frierson and Durran (with the help of some students) cooled aluminum cans filled with water at close-to-freezing temperature in an ice-water bath. The cans were equipped with special snap-on tops that kept air sealed in and allowed the researchers to insert a digital thermometer to record the water's temperature.
The cans were then put in a temperature- and humidity-controlled chamber for five minutes, taken out, and had their temperatures taken again.
Their experiments showed that humidity made a difference in how quickly the cans warmed up. When cans were kept at 77 degrees Fahrenheit for five minutes, cans that experienced a relative humidity of about 40 percent heated up by about 39 degrees; at a relative humidity of about 85 percent, the temperature of the water inside the can jumped 44 degrees in the same time period.
“Cold drink cans warm up significantly faster in hot, humid locations than in hot, dry locations -- by approximately a factor of two in typical summertime weather conditions,” Durran and Frierson wrote.
Based on their results, Durran and Frierson estimate that the worst place to try to keep a beverage cool would be in Dhahran, Saudi Arabia. In July 2003, Dharhan saw a day where the temperature climbed to around 109 degrees Fahrenheit, with a record dew point of 95 degrees Fahrenheit -- an extremely humid day! (For reference, people usually start noticing that the air “feels sticky” when the dew point reaches 65 degrees Fahrenheit.)
That kind of humidity would cause some serious condensation. Under those conditions, a beer kept at a temp near freezing would be warmed up by 50 degrees Fahrenheit within five minutes, the pair wrote in Physics Today.
The heating power of condensation has ramifications beyond your enjoyment of a frosty beverage. The heat transferred when masses of water condenses or evaporates plays a big role in the Earth's weather, and climate change is expected to tip the scales towards the humid side.
“We expect a much moister atmosphere with global warming because warmer air can hold a lot more water vapor,” Frierson said in a statement.