Was it only six months ago that we heard the phrase “It’s not the heat, it’s the humidity”? The reason “it’s the humidity” is because of the way we cool our bodies.

When we get hot we sweat. That water is evaporated from our skin and that is a cooling process. However, the water has to go into the air and if the air already has a lot of water, i.e. humidity, the sweat doesn’t evaporate and you are hot. The temperature combined with the relative humidity determines what we call the “heat index”, or how hot it feels.

Right now I would love a hot, humid day because what we are hearing now is “wind chill”. Wind chill in the winter is like “heat index” in the summer. It is a real thing, however, you can’t measure it! Nope, you have to calculate it. That takes math.

Wind chill is not a measure of temperature, it is a measure of the effect that temperature and wind combined have on your skin. In fact, it was the question about that combined effect that led two U.S. weather researchers to explore it.

Everyone knew that it felt colder when the wind blew but Paul Siple and Charles Passel decided to quantify how much colder the wind made it. They had a strong desire to find out because they were stationed in Antarctica and there was concern as to how long a worker could be outside before they risked frostbite. Siple and Passel first studied the effect of wind and temperature on freezing water in plastic containers and, yes, the wind did have a significant impact on the time it took the water to freeze.

From that they developed a formula that they published in 1945 that estimated the effect of the combination of wind and temperature on human skin.

The National Weather Service revamped Siple and Passel’s formula in 1973 and then again in 2001. What they discovered was that the 1943 and 1973 formulas overestimated the negative impact of the wind and temperature because the temperature of the air is typically taken at six feet above the ground and the wind speed at 10 feet above the ground. The wind speed at six feet is significantly lower than at 10 feet, and thus the wind chill is as well.

However, just altering the formula wasn’t enough. Researchers in Canada wanted to measure the impact of the wind and cold on living people, not bottles of water! So in 2000, 12 volunteers, including the lead researcher, subjected themselves to walking in a wind tunnel for 90 minutes at a time at various temperatures and wind speeds. One of the tests also included someone throwing water into their faces every 15 seconds to look at the impact of rain or snow along with the wind and temperature. The formula determined from those experiments is what we use today.

Under that formula, Saturday night’s -19 temperature with a 15 mph wind gave us a wind chill of -45. Because “wind chill” isn’t a direct measure you shouldn’t see a degree symbol with the number. At -45 wind chill frostbite can occur in 10 minutes or less! So, while you can’t measure wind chill directly, it is real and can be a very important number to pay attention to!