As the weather warms and the outdoor picnic/barbecue season commences, there are few more important questions than how can I keep my drinks chilled? And for the scientifically minded: what processes control the temperatures of my cold beverages?
Relax! Cutting edge research at the University of Washington reveals the answers to these pressing issues. It turns out that humidity can be as important as temperature in warming your drinks, and that a cold brew is just an inexpensive koozie away.
In a recently published paper in Physics Today, UW Professors Dale Durran and Dargan Friesen, aided by undergraduates Stella Choi and Steven Brey, did a series of experiments with chilled soft drink cans exposed to various temperatures and humidities.
In their experiments they started with 12oz cans filled with water at a temperature just above freezing. Then they exposed the cans to various temperatures and humidities over a five-minute period. The results were sobering. Condensation counts.
When you take a can out of the frig or cooler, the can and its contents warm by two processes: conduction of heat to the can (this is called sensible heating) and by condensation of water vapor on the can (called latent heating). When water vapor condenses it releases huge amounts of heat, specifically 680 calories per gram of water condensed. So when you see that shiny, watery condensation on your cold drink, imagine those drops as powerful heaters!
For a typical Seattle summer day (77F) and moderate humidity (say 50% relative humidity) the UW experiments showed that the drink warmed up by roughly 9F in five minutes and about 2.5F of the warming was due to the condensation. If the humidity was 80%, a rare occurrence here in Seattle, the can would warm up by 11F and about 6F would be due to condensation. You don't want to enjoy your beer or soft drink in a hot shower at 100% relative humidity! Few of you do that I suspect.
They did the same experiments at 95F and a range of humidities; the impacts of humidity became HUGE at high temperatures and high humidities. At 95F and 80% relative humidity the can warmed by 22F, and 13F of the warming was by contributed by condensation.
No wonder this research became a viral subject in India and Pakistan, which extreme heat and humidity are commonplace.
So what does one do about this scourge? One that robs our enjoyment of a frosty drink on a warm summer's day.
Easy! Buy an inexpensive koozie that fits around a can or bottle. They are cheap and effective devices (see images) that act as insulators, which helps in two ways. First, they lesses the conduction of heat from the surrounding air to the can and bottle. But just as important, it pretty much stops the condensation, since the outside of the koozie generally does not cool enough to cause condensation (the outside of the koozie stays about the dew point of the surrounding air).