Monday, July 15, 2013

Physics of a Beer Koozy

Condensation beads on a cold beer bottle in upstate NY.
Beer ~40ºF.  Dewpoint ~70ºF
During the dog days of summer, it's critical to keep your drinks cold.  When the dewpoint is high, such as during monsoon surges, you either drink fast or use a beer koozy, as demonstrated above.  Here's why.

During situations with high dew points, condensation will form on a cold beer (or soft drink) can or bottle.  The rate at which your beer or soft drink warms is thus affected not only by the direct flow of heat from the air to the bottle, but also condensational warming.  This warming is the result of a net flow of water molecules from vapor to liquid, which releases what meteorologists call latent heat (the converse of this is evaporative cooling, which you have certainly experienced on dry, windy days in Utah).

How important is condensational warming?  This all important issue was recently explored by University of Washington Atmospheric Scientists Dale Durran and Dargan Frierson in an article published in Physics Today (Dale is a former Professor of Meteorology at the University of Utah).
"At 35 °C and a relative humidity greater than 60%, the temperature rise due to latent heating exceeds that due to heat transfer from dry air: Latent heating is the dominant factor warming your cold beer." - Durran and Frierson (2013)
In US units, that equates to a temperature of 95ªF and a dewpoint of about 78ºF, far steamier than we see in Utah [the warming is measured for a 5 minute period, details here, but it may be paywalled for those without a Physics Today license.].  If we picked something like 95ºF with a relative humidity of 35% and a dewpoint of 63ºF, closer to the steamiest weather we get in Utah, their Fig. 1b suggests perhaps 20–25% of the initial warming of your beer is due to condensation.

Beer koozies provide insulation that slows the flow of heat from the air to the bottle, but they also prevent condensation.  Note in the photo above that there is no condensation on the koozy.  This can be especially important in hot, high humidity environments where the warming of your beer or soft drink is strongly influenced by condensation.  In Utah, the condensational warming effect is often negligible (when it is dry and you don't see "sweat" on the container holding your drink), but it can be an added warming factor during monsoon surges and other periods with elevated dewpoints.

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