What is wind chill and why does it make us feel colder?
We’ve all been there on a cold winter’s morning – standing at the bus stop with a bitter wind blowing hard, giving a numbing chill to our extremities.
With the weather set to turn notably colder in the coming days – especially in the brisk northerly wind – I thought I’d take a look at what wind chill is and why it makes us feel colder.
What is wind chill?
Wind chill is a description of how the air around us feels when a wind of a given strength is combined with a particular air temperature.
The most common way that you may have seen this expressed on TV or online is as a ‘feels like’ temperature.
However, there is often confusion about how this works and why the temperature on a thermometer doesn’t ever read the same as a given feels like temperature.
To understand the reason for this, we need to look at how the human body works and interacts with the air around it.
Wind chill and the human body
The human body has a natural mechanism by which it regulates temperature to try and keep cool in summer and warm in winter.
This involves the presence of warmth and moisture on our skin which is in contact with the air around us.
In summer when it is hot, our bodies sweat in order to produce more moisture on our skin that can be evaporated by the air around us. Evaporation is a cooling process, so as the moisture on our skin evaporates, it lowers body temperature – allowing us to cool down.
In winter, we obviously don’t sweat when it’s cold, but there is still moisture present on our skin.
As the wind blows over our skin in the depths of winter, this moisture evaporates and, like in summer, has a cooling effect causing body temperature to lower.
However, given that it is undesirable for our bodies to get even colder when it is already very cold outside, we need to protect ourselves with warm clothing.
How is wind chill calculated?
There is no global definition of wind chill and how it is calculated and described around the world varies from place to place.
However, here in the UK, along with the US and Canada, a system called the Joint Action Group for Temp Indices (JAG/TI) is used to realistically measure wind chill.
This estimates wind chill by measuring how much heat is lost from a person’s bare face at a walking speed of 3mph, taking into account the air temperature and wind speed.
As you would imagine, the stronger the wind at a particular air temperature, the greater the rate at which heat is lost from our faces, resulting in a more marked wind chill.
So, when you’re standing at the bus stop in the brisk northerly wind in the coming days, you’ll know exactly why the wind makes it feel even colder.