What causes the phenomenon known as the wind chill factor?

AUSTIN, Texas - Charles Dickens wrote in his novel Great Expectations "It was one of those March days when the sun shines hot and the wind blows cold."

We know that feeling. Today as temperatures hover in the upper 50s with a steady 15-25 mph wind from the northwest, the wind definitely blows cold.

Meteorologists have quantified that cold feeling into a metric called the ‘wind chill factor’. It’s also called the ‘feels like’ temperature which can be lumped in with the heat index on the other side of the scale.

So what is the wind chill factor? Well, according to the National Weather Service it is [35.74 + 0.6215t - 35.75(𝑉0.16) + 0.4275t(𝑉0.16)]. Which is a lot of numbers. Meteorologists tend to like math.

It basically means the wind chill temperature is directly proportional to both the wind speed and actual air temperature. Essentially, if it’s frigid out with a light wind, the light wind will impart a greater ‘wind chill’ factor than if it was not as cold. Same thing goes with merely cold or chilly temperatures and very strong winds.

For this reason, the wind chill index is only calculated at temperatures of 40° and cooler. Anything above 40° doesn’t impart enough of a difference in ‘feels like’ temperature. Plus temperatures above 40° aren’t as dangerous as temperatures below.

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The real impacts aren’t really felt until temperatures get into the mid 30s and colder. For example, the National Weather Service says that if the temperature was 35°F a 5 mph wind would make the wind chill feel like 31°. Not that big of a difference. But at 0° F that same 5 mph wind would make it feel like -11°!

Similarly for strong winds but comparatively mild temperatures. A temperature of 35° with a 60 mph wind feels like 17°F. That same wind at 0°F feels like -33°! Thank god we live in central Texas!

So what causes this phenomenon? It’s ultimately down to the fact that air is a good insulator if that air stays close to the heat source. A thermos, or indeed home insulation, only works if the air trapped in it stays where it is. Wind removes the air surrounding your body, effectively pushing your heat away into the atmosphere. The replacement air is colder and needs your body to warm it up but often gets pushed away by the wind.

That process of your body warming the air around you is why you feel cold as heat is transferred from your body to the air. And if it’s windy it all gets blown away before you can heat it up.

It is important to note that the wind is trying to cool you down to the ambient air temperature as quickly as it can. An object will never fall to the wind chill temperature. But it will get to the air temperature faster with a stronger wind. That process is even faster with colder temperatures since the temperature difference is greater.

This can be very dangerous since the removal of heat from your body can result in hypothermia. Or, if the air temperature is below freezing, frostbite.

That’s why it’s so important to wear extra layers in the cold. The layers of clothes trap more air close to your body, insulating you from the cold air outside. That’s doubly important on days and nights when the wind is strong.

FOX 7 Austin Meteorologists update the forecast on-air, online, and on social media. You can also receive updates by downloading the FOX 7 Austin WAPP.