Take a T-shirt out of a bucket of water, carry it outside at −20°F and wait. Envision Sports Designs of Dubuque, Iowa, filmed exactly that, and within about a minute the soaked cotton had turned stiff. The clip spread during the brutal cold wave of January 2014, when the reported wind chill in Dubuque sat around −50°F.
Why a wet T-shirt freezes so fast at −20°F
An air temperature of −20°F equals about −29°C, nearly 30 degrees below the freezing point of water. A T-shirt is an ideal object for showing what that means:
- Thin layers of water. Soaked cotton holds water spread across a huge surface of fine fibres, so there is very little depth for heat to escape from.
- Wind. Moving air strips heat from the wet fabric much faster than still air, so the shirt reaches freezing quickly.
- Evaporation. Some of the water evaporates, and evaporation carries away extra heat.
As the water between the fibres turns to ice, it locks the fabric in whatever shape it was held. That is why the shirt goes rigid: the ice, not the cotton, is holding it up.
Wind chill explained: −20°F air, −50°F feel
The US National Weather Service defines wind chill as how cold people and animals feel outdoors, based on the rate of heat loss from exposed skin caused by wind and cold. Its current formula, adopted in 2001, is:
Wind chill (°F) = 35.74 + 0.6215T − 35.75(V0.16) + 0.4275T(V0.16), where T is the air temperature in °F and V the wind speed in mph.
Plug in −20°F and the numbers line up with the Dubuque report. A 10 mph wind gives a wind chill of about −41°F; at 20 mph it is about −48°F, and at 25 mph about −51°F. In Celsius, that is air at −29°C feeling like roughly −46°C.
One detail surprises many people: wind chill applies to warm bodies losing heat. Objects such as a car radiator, a water pipe or a wet T-shirt are cooled by the wind only down to the actual air temperature. The wind makes the shirt freeze faster, but it cannot make it colder than −20°F.
The January 2014 cold wave in the Midwest
The demonstration came from one of the harshest North American winters in decades. Around 2 January 2014 an Arctic front swept south across Canada and the United States after a sudden stratospheric warming event disrupted the polar vortex.
- Babbitt, Minnesota, recorded −37°F on 6 January, the lowest temperature in the country that day.
- Chicago’s O’Hare Airport fell to −16°F, and Minnesota closed every public school in the state for the first time in 17 years.
- Airlines cancelled about 20,000 flights after the storm began, and about 91 percent of the Great Lakes eventually froze over.
- Iowa went on to record its ninth-coldest winter in 141 years.
Staying safe in extreme cold
Treat wind chill as a guide for how long skin can stay exposed. The NWS notes that at 0°F with a 15 mph wind, the wind chill is −19°F and exposed skin can freeze in 30 minutes; at Dubuque’s −50°F the margin is far shorter. Cover your face, hands and ears, dress in layers and keep clothing dry, since wet fabric loses its insulating value just as the T-shirt shows. Bright sunshine can make conditions feel 10 to 18°F warmer than the wind chill figure, which assumes no sun.
For more cold-weather science, compare interpreting a cold-weather visualisation of the same winter, and see freezing a much thinner soap film.
Related reading
- An Ice Lens and the Concentration of Sunlight
- Yellowknife’s Hot-Water Cloud in Extreme Cold
- A Wisconsin Cold-Weather Stunt in Its Original Setting
- An Ice-Covered Stairwell: The Photographs Behind a Temperature Claim
- New York’s January 2014 Snowstorm in Time-Lapse
Sources
- You Won't Believe It! T-shirt Freezes in 1 Minute! – envisiontees on YouTube
- National Weather Service: Wind Chill Chart
- Early 2014 North American cold wave – Wikipedia
- Photo and video: Envision Sports Designs.
No comments