Have you ever wondered why do windy winter days seem so bitterly cold? A winter day with a strong wind feels much colder than the one with a mild wind. However, the air temperature remains same. This cooling is promoted by air circulation. It removes the warm air blanket surrounding our body, which acts as a potential insulator for protecting heat loss. A similar effect can be felt when you step into cool water. After standing a short time in the water, you will notice that the water next to your body seems warmer. Sudden movement sweeps away the warm layer of water and you again start feeling cold. Additionally, due to air circulation, the process of evaporation takes place promoting the coolness around. It is more effective in dry and warm air.
Strong winds can make the environment chilly although some amount of cooling is necessary for comfort. The wind chill factor or WCF expresses the rate at which the heat is lost from our exposed skin due to cold wind. With an increase in the speed of wind, heat is taken away rapidly from our body thereby bringing down our body temperature. The wind chill factor is successfully used to measure the probability of hypothermia or frostbite. This factor also affects warm inanimate object. For example: Take two glasses filled with 100-degree water. Place one glass inside the refrigerator, and leave one outside. The air temperature is 35 degrees and the wind is blowing at 25 mph. Hence the wind chill makes it feel like 8 degrees. The glass outside will get cold quicker. However, the glass outside will not get colder than 35 degrees whether the air is blowing or not.
Paul Allman Siple (an American Antarctic explorer and geographer) with Charles F. Passel developed the wind chill factor, coined the term. There are two systems used for calculating the wind chill factor. The Canadian factor gives figures of heat loss in watts per square meter whereas the American National Weather Service gives results in the form of equivalent temperature. The National Weather Service of the United States regularly warns people about the possibilities of wind chills. While calculating the wind chill factor it is important to use the index in a qualitative manner as these calculations were initially created from the experiments in the 1940s performed by Siple and Passel during their stay in Antarctica. The duo used this factor to measure the amount of time taken by water to freeze at different temperatures and wind speeds.
Wind chill factor is termed so because it is the temperature we feel due to the wind. This can be explained with the given example. If the reading on a thermometer is 35 degrees Fahrenheit, while the wind speed is 25 mph then the wind chill factor makes us feel as if the outside temperature is 8 degrees F. This is so because our 98-degree body loses heat as if it is 8 degrees outside. This process of loss of heat due to the blowing of the wind is due to convection. This is the same process which cools down your soup when you blow over hot soup. Warm blooded animals are provided with natural insulation in the form of hair or feathers, which act as poor heat conductors. Hence the loss of heat in these animals is restrained as air gets trapped in between the hair or feathers. Humans protect themselves by wearing warm clothing made from animal skins, feathers or synthetic materials that carry similar features.
The new formula was adapted by the National Weather Service and the Canadian weather in 2001 fall. Two formulas were developed- one for Celsius reading and the other for Fahrenheit temperatures. These new formulae are developed on the basis of scientific knowledge that has evolved from the experiments carried on faces of volunteers when exposed to different temperatures and wind combination in a wind tunnel. The new formula for winds in mph and Fahrenheit temperatures is:
Wind chill temperature = 35.74 + 0.6215T - 35.75V (**0.16) + 0.4275TV(**0.16)
In the formula, V is the wind speed in statute mph, and T is the temperature in degrees Fahrenheit.