It's that time of the year again where the wind chill is something you'll get use to hearing. While the thermometer outside may say it's 43, you'll hear your local meteorologist say it feels like it's actually in the 30s. So, what is wind chill and how is it calculated?
You may be surprised to hear that wind chill is actually a recent invention that dates back to 1945. Two Antarctic explorers by the name of Paul Siple and Charles Passel were measuring how wind speeds affects the rate of heat loss. They did this by doing a simple experiment using a container, water, and a pole. The results showed that the windier it is the faster water turns to ice, which means for humans the colder it feels.
The science behind why it feels cold is actually really simple. We lose about 15 percent of our body heat through convection (similar to sitting in front of a window or having a fan blow on you). This heat loss creates a boundary between our skin and the cooler air. However, when you factor in windy days, the wind breaks up that heat layer and causes us to lose heat significantly faster, therefore, making it feel colder.
The wind chill formula has been modified over the years, most recently in 2001 by The Joint Action Group for Temperature Indices. It was through an experiment that they found a more accurate way of depicting wind chill values. Today our wind chill index is based on the wind speed at five feet in the air, which is the average face height.
Of course, wind chill is very subjective and will always be the topic of debate. A lot of how it feels outside is dependent upon body type, the type of clothes you have on, your rate of heat loss, among a variety of other things.
One thing is true though, if the air temperature is 32 or above, even if the wind chill is below freezing, water won't freeze, neither will your face.
Do you want to calculate your own wind chill? Here's how to do it.
WindChill = 35.74 + (0.6215 × T) − (35.75 × V^0.16) + (0.4275 × T × V^0.16)
Where V is wind speed and T is temperature.
I.E. Suppose you have a temperature of 45 and a wind speed of 26 mph. You would plug it into the formula and get:
Windchill = 35.74 + (0.6215 x 45) - (35.75 x 26^0.16) + (0.4275 x 45 x 26^0.16)
= 35.74 + 27.9675 - 60.2104 + 32.3999
= Wind chill of 35.9 degrees.
The wind chill index calculates the "feels like" temperature per the NWS through:
* Wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings form the national standard height of 33 feet, typical height of an anemometer
* Is based on a human face model
* Incorporates heat transfer theory: heat loss from the body to its surroundings, during cold and windy days
* Lowers the calm wind threshold to 3 mph
* Uses a consistent standard for skin tissue resistance
* Assumes no impact from the sun.
The National Weather Service says that the wind chill temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the wind chill temperature by 10 to 18 degrees.