Image Courtesy: Peter Griffin

How is Wind Chill Calculated?

By Paul Trambley @yourmetpaul December 5, 2016 10:32 pm CST

Prior to the 1970's, the term wind chill had not yet made it into the vernacular for meteorologists in the United States. In 1973, the wind chill temperature first began to be used and accepted into the field of meteorology. Today, wind chill values are given so much emphasis that the public can sometimes have a hard time deciphering which temperatures given are wind chills and which are actual air temperature readings. Some folks think that the use of a wind chill temperature is just a sensationalized number to grab folk's attention. Others feel it is an important reading for folks to be aware of, as it tells a more complete story of what it really 'feels like' outdoors to our bodies.

The basic science behind the wind chill involves the fact that our bodies are constantly warming the air that resides right next to our skin. When there is little wind, our bodies are able to keep this semi-insulated shell of air around it, as we expose ourselves to the outdoors. During windy conditions, our body continues to warm the air next to us, but that body-warmed shell of air is continually transported away from us, thereby making it feel colder to our bodies. This consistent heat loss results in colder skin temperatures in windy conditions compared to that found during calm conditions.

A Little Wind Chill History Lesson...

Anyone who has been outdoors during a cold day with light winds and another cold day with gusty winds, knows that wind certainly does make it feel colder to our bodies. This was also apparent to polar explorers who were on an expedition in Antarctica back in 1939. A group of 125 men went on an expedition, with orders from President Franklin D. Roosevelt, to establish two military bases on the icy continent. It was during this mission that two American scientists, Charles Passel and Paul Siple, noted the huge difference in how cold they felt based on how much wind they encountered on the expedition. They conducted experiments that started with a cup of water of a specific temperature. Then they would expose this cup to the elements and record how long it took the water to freeze. They did this in light and heavy wind conditions, and noted how the water froze quicker as the wind increased. This occurred due to the fact that higher winds would transport heat away from the water quicker, thereby freezing it faster. This data was then crunched over time to make the first formula for calculating the wind chill. 

T(wc) = 0.0817(3.71V^0.5 + 5.81 -0.25V)(T - 91.4) + 91.4

V=Wind Speed

T=Temperature

^ = raised to a power (exponential)

Calculating windchill

Image Courtesy: Government of Canada

The Wind Chill Revolution

This first formula was accepted for decades, until scientists began to think of how a better formula could be made. You could call it a 'wind chill revolution'. It became widely thought among meteorologists that this old system was overstating the wind chill, as wind chill readings seemed to be rather inflated in this outdated system. The reasoning for this was that they believed that our bodies probably don't act exactly like a glass of water in the cold, and a more relevant set of data was needed to formulate a revised algorithm.  Two North American scientists (Randall Osczevski and Maurice Bluestein) collaborated to create a new wind chill formula.  The new formula was introduced and replaced the old windchill formula in the fall of 2001. The new formula used updated heat transfer theory and actually used humans in clinical trials that were conducted. 12 volunteers were used in the experiments that involved walking on a treadmill in a refrigerated wind tunnel (now that sounds like a great time)! Sensors affixed to the face took measurements of skin temperature at various points in the experiment. With the data collected during these trials, a revised chill formula was born. 

Windchill (ºF) = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)

Where: T = Air Temperature (F)
V = Wind Speed (mph)
^ = raised to a power (exponential)

http://www.noaanews.noaa.gov/stories/images/windchillfactor-new.jpg

Image Above shows the wind chill reading comparison of the new and old wind chill formula when the temperature is 5F

Whether you prefer the old or new wind chill formula, the end result is that wind certainly makes a big difference in how cold we feel when we are outdoors. The best way to mitigate the impact of the wind chill is dressing in layers. By layering, you allow your body the ability to retain some of the heat that would otherwise be lost due to wind. Wearing gloves and insulated socks/footwear is also important, as our extremeties are the most vulnerable to frostbite. In extreme cold, it is also advisable to wear a scarf or balaclava, in order to keep any exposed flesh to a minimum. By taking these precautions, you will greatly lessen the risk for frostbite. If you are outside and do start to notice numbness or a prickly feeling on your skin, you are experiencing the initial signs of frostbite. At that point, you would want to seek warmth immediately if possible, in order to avoid full-fledged frostbite. The key is to be smart and don't let the efforts of those 12 brave volunteers who subjected themselves to varied elements go to waste!

Image result for windchill chart

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