What’s with the wind chill?

Graphic courtesy Pericoli

First things first, we’re not expecting any wind chill issues this weekend, but we’ll get there eventually. I had an interesting viewer question:

Good evening,

This afternoon’s walk with my dog reminded me of an issue that I have been questioning. On the 4:30 weather report, the man talked about the uncomfortable heat index in the upper 90s.

At 5:30 when I started our walk, the app on my cell phone said 88 degrees with a “feels like temp of 94. Actually, it felt very nice because of a wind estimated at 13 mph by my app. There has been a fairly strong wind all week which has made walks in the middle of the afternoon quite tolerable.

This begs the question, why do weather forecasters only use wind during the winter to come up with the “wind chill factor” and ignore it during the summer when it might actually lower the “feels-like temperature?”

Regards,

Bob

I guess I am ‘the man’ Bob is referring to, but this is a legitimate question I’ll get to in a second. First, here is how we got here in the first place: scientists Paul Siple and Charles Passel in the Antarctic during the 1940s studied how quickly water froze in plastic cylinders considering wind speeds: they found that the faster the wind, the quicker the temperature lowered allowing freezing. This led to how quickly the temperature of human skin would lower given a certain wind speed and wind chill equivalent temperatures were created. They published their formula in 1945.

Charles Passel is on the left with other scientists. More than a hundred dogs pulled equipment across the snowy landscape. Image credit: Byrd Polar and Climate Research Center Archival Program, via Tennessee Aquarium Blog.

Their original charts were not put in use until 1973 and remained in place until 2000 when Canadian and American scientists took into consideration other wind chill factors such as the height the winds were measured (closer to the ground where we walk and thus less affect than the original), the average speed we walk (3 mph), even what a splash of water on the face might do. A multitude of different experiments at different temperatures and wind speeds ensued to produce the current wind chill table:

This chart doesn't even really get going until the temperature fall to 40F

You can read more about the background and testing from WeatherUnderground here and keep in mind that the chart doesn’t even consider full sunshine but that warmth will counteract wind chill effects by at least 10 degrees. The reason for the 50F threshold is that seems to be the temperature at which human skin will feel ‘pain’ although even that is subjective. From the NWS:

The NWS Wind Chill Temperature (WCT) index uses advances in science, technology, and computer modeling to provide an accurate, understandable, and useful formula for calculating the dangers from winter winds and freezing temperatures. The index does the following:

  • Calculates wind speed at an average height of 5 feet, the typical height of an adult human face, based on readings from the national standard height of 33 feet, which is the typical height of an anemometer
  • Is based on a human face model
  • Incorporates heat transfer theory based on heat loss from the body to its surroundings, during cold and breezy/windy days
  • Lowers the calm wind threshold to 3 mph
  • Uses a consistent standard for skin tissue resistance
  • Assumes no impact from the sun, i.e., clear night sky.

Wind Chill Warnings and Advisories are subjective and here in Houston we do not issue them unless the Wind Chill is expected to fall to 13F (Advisory) or 0F (Warning).

SO WHAT ABOUT ANIMALS?

The Wind Chill effect DOES apply to animals. In fact, one group I found, the Pericoli group specializes in ‘ideal climate’ and claims that an 85F degree temperature with a 5 mph wind can actually make an animal ‘feel’ cooler by ten degrees. Here are three factors they consider:

  • The age of the animals is very important. Younger animals are smaller in size, have more space between them and their heat dispersion is clearly easier. As a result, they need a smaller and less powerful airflow.
  • Many wind chill charts refer to a precise temperature, but this is not correct at all. The temperature fluctuations must be taken into consideration: as the temperature drops, the wind chill effect increases because there is a greater degree difference between the air and the animals.
  • Recent studies have placed new emphasis on fan maintenance, keeping the shutters system clean and the belt properly tensioned.

Perhaps we will one day see a calculation (and ensuing chart) that considers the wind cooling effect on humans and animals at any given temperature--sounds like a perfect A&M thesis to me!

Hope that helps answer your question, Bob, and here’s to a breezy, cool weekend!

Frank “the man”

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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.

Amanda Cochran is an Edward R. Murrow award-winning journalist. She specializes in Texas features, consumer and business news and local crime coverage.