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Wind Chill Facts

Brrr! It’s cold outside! Time to bundle up in layers to keep that cold winter chill at bay.

But, how many layers will I need?

The temperature is cold enough, but add in the wind chill factor, and now I will need to make sure I don’t get frostbite or allow hypothermia to set in.

But, what exactly is wind chill, and why is it so dangerous?

When do I need to start paying attention to the negative numbers on the weather map and the warnings that the meteorologist is announcing like it’s breaking news?

Wind Chill Facts for Kids

  • Wind chill is the feels like temperature
  • Wind chill is a mathematically derived number
  • Frostbite and hypothermia are dangers of the cold
  • Wind chill impacts only living things
  • Cold with wind can be dangerous
  • Avoid wind chill by dressing in layers
  • Wind chill warning is issued at -25 F or below
  • American explorer devised windchill calculations

What is wind chill?

Simply put, wind chill is the “feels-like” temperature. It is how the temperature actually feels to humans. It only applies to people, animals, and other living things and does not affect inanimate objects.

What exactly is wind chill?

As mentioned, wind chill is how the temperature feels. In other words, it’s how humans perceive the temperature to be. In more technical terms, wind chill is the cooling effect of wind blowing on a surface, like human skin. It is the cooling effect of wind and temperature combined. The wind chill is based on how much heat is lost from exposed skin in cold and windy conditions. It is the decrease in air temperature felt when the wind touches the human body or skin.

Why is wind chill important?

Besides the fact that we will feel colder when the wind chill is low, wind chill is important to be aware of because the lower the wind chill temperature, the more likely we are to develop frostbite or hypothermia. Hypothermia happens when the human body’s core temperature falls below 95 degrees F.

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Facts for Kids

What is the wind chill factor?

Think of it this way. When we try to eat a very hot bowl of soup, our first reaction is probably going to be to blow on it to cool it down. As we begin to blow on the soup, it cools down quickly because the air is moving.

In more scientific terms, moving air increases the soup’s loss of heat. Regardless, the soup begins to cool down more rapidly when we blow on it. This concept that moving air creates a cooling effect is the basic idea behind what a wind chill factor is.

Can wind chill kill you?

Since our bodies naturally radiate heat, with the normal human body temperatures typically hovering around 98.6 degrees F, when a cool breeze blows on the skin, it starts to cool down the human body. If we are out in the cold for long periods of time, that cold air starts to lower the body’s internal temperature.

This leads to more than just feeling cold. It can lead to hypothermia and frostbite. Hypothermia sets in when the body’s core temperature falls below 95 degrees F.

Scientists are not even quite sure how wind chill actually affects the body, but they think it has something to do with the brain sending a message to blood vessels telling them to shut down so more blood can flow to the core of the body, keeping major organs warm, so we can stay alive and not literally freeze to death.

According to the National Weather Service, NWS, we cannot rely on just the wind chill value alone. We must also consider the wind speed.

For instance, because of the wind’s ability to remove heat, if the temperature is -40 F and wind speed is 5 mph, frostbite can occur in as little as 10 minutes or less. Yet, at -5 F, frostbite is still a threat in the same amount of time with a wind speed of 35 mph.

When wind chills are in the negative 15 to negative 20-degree F range, frostbite will occur in 30 minutes time.

The NWS also reports varying degrees of frostbite that range from first to fourth degree, with fourth-degree being the worst danger, as follows:

  • First-degree frostbite – The surface of the skin is frozen, referred to as frostnip.
  • Second-degree frostbite – The skin may harden and freeze, with blisters forming in a day or two.
  • Third-degree frostbite – Muscles, tendons, nerves, and blood vessels begin to freeze.
  • Fourth-degree frostbite – Skin may become dark blue or even black and painful with pain that lasts for more than a few hours. There is a danger of gangrene that would require amputation of extremities.

When are wind chill advisories or warnings issued?

For a wind chill to go into effect, temperatures must be at or below 50 degrees F and winds much be more than 3 mph. Obviously, 50 degrees won’t set off a warning, but that is the starting point at which wind chill is measured.

Each weather service uses different criteria that trigger issuing warnings and watches. Typically, a wind chill warning is issued when very cold air and strong winds could cause frostbite or hypothermia in a matter of minutes.

A watch is when there is a chance of very cold air and strong winds. An advisory is typically issued when cold and breezy conditions could cause frostbite or hypothermia.

Does wind chill actually make it colder?

While blowing cold air will make the temperature feel a lot colder, wind chill does not actually change the temperature. It merely gives the impression that the air temperature is colder.

Studies have been conducted using two thermometers. One thermometer is left in the weather with no blowing air, while the other is placed in a breeze or with the wind blowing on it. After several minutes, both thermometers read the same temperature. 

Think of it in terms of riding in a vehicle that shows the outside temperature on the dashboard. The temperature display will not change regardless of whether we are traveling at 60 mph or sitting at a stoplight not moving.

However, if we are outside in the weather elements, the blowing cold air will cause the human body to cool off faster even though the outside temperature is constant. This rapid cooling process caused by the blowing cold air can lead to health issues such as hypothermia.

How is the wind chill factor calculated?

American explorers Paul Siple and Charles Passel originated the concept of calculating wind chill. Their method of figuring out wind chill is referred to as the Siple and Passel Index.

They based their calculations on experiments they conducted while on a Polar expedition in the 1930s. Their formula for wind chill is calculated by using a complicated looking formula that looks like this:

35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)

Where:

T = air temperature (F)

V = wind speed (mph)

Another method was developed by Robert G. Steadman in 1971, who based his method on the amount of clothing needed to protect ourselves from frostbite. This method is called The Steadman Windchill

The Steadman wind chill equation for Fahrenheit temperatures is:

wind chillSF = 3.16 – 1.20V + 0.980T + 0.0044V2 + 0.0083(V T)

where V is the wind speed in statute miles per hour

and

where T is the air temperature in degrees Fahrenheit

The NWS National Weather Service provides a great Wind Chill Chart that does the math for us. All we need to make the calculation is the actual temperature and wind speed.

A more recent formula that was developed in Canada is called the Joint Action Group for Temperature indices, JAG/TI method.

The JAG consists of the NWS, Meteorologic Services of Canada MSC, the academic research community of Indiana University-Purdue University in Indianapolis IUPUI, University of Delaware, University of Missouri, and the International Society of Biometeorology.

According to the Joint Action Group on Weather Indices, this method is based on the heat transport from the body to the skin and is closer to the human experience of heat loss than other methods. America, The UK, and Iceland use this method.

The current wind chill temperature WCT index formula was developed during the early 2000s and helped to standardize the Wind Chill Index as well as uses computerized technology. It has been used since the winter of 2001-02. Some factors going into the WCT Index include:

  • Wind speed that is calculated at the average height of a human body’s face, 5-feet as opposed to what had previously been used at the standard anemometer height of 33 feet
  • Incorporates modern heat transfer theory that takes into consideration heat loss from the body to its surroundings during cold and breezy or windy days
  • Lowers the calm wind threshold to 3 mph
  • Uses a consistent standard for skin tissue resistance
  • Assumes the worst-case scenario for solar radiation, which would be a clear night sky