Frost combined with an icy wind: we have all experienced it. At these moments, the wind chill temperature is what matters most. It tells us much more about what your body truly experiences than what the thermometer indicates. Extreme cold can pose a significant physical threat to our body. That is why the KNMI issues warnings, for example with a yellow code when wind chill temperatures fall below –15 degrees Celsius. In a warming world, we may become less accustomed to such cold, making the need to warn potentially even greater.
Extreme cold as a threat to our body
Issuing warnings for extreme cold is an important task. Low temperatures combined with strong wind can cause our body temperature to drop. More heat is lost than produced. When our body temperature falls below 35 degrees Celsius, hypothermia occurs. Shivering, reduced fine motor skills, and in severe cases loss of coordination and consciousness are possible. Especially vulnerable groups like children and the elderly have a higher risk of cold injuries. Worldwide, more people still die from extreme cold than from heat. A 2024 study concluded that between 1991-2020 in Europe, eight times more people died from cold (363,809) than from heat (43,729). However, it is expected that heat-related deaths will increase more than cold-related deaths decrease due to further warming. Total temperature-related mortality will thus rise.
The wind chill temperature is much more relevant in cold combined with wind. The thermometer may say something very different from what we experience outside. How it feels outside determines whether we go for a walk or do other outdoor activities. Emergency services and outdoor workers can better respond to winter risks using wind chill temperature.
How cold does it feel?
Our body loses heat by (1) radiating heat, (2) air or water removing heat, or (3) contact with colder objects. During cold conditions, wind is the main factor blowing away the warm air layer around our skin. This makes it feel colder than the thermometer shows.
The difference between temperature and wind chill can be seen as a measure of heat loss caused by wind. Wind chill, often called wind chill in English, is not a temperature we can measure. We calculate it using measured temperature and wind speed. KNMI uses the JAG/TI method to calculate wind chill (figure 1). This method balances heat loss and production. It assumes a healthy, adult, walking person of average height, but experience may vary. Sun radiation is not considered.
Unlike in summer, humidity plays a negligible role. However, when we have sweaty skin or wear wet clothes, we lose heat faster because water conducts heat better than air. This is why the layering principle works: thin air layers between clothing retain heat. Evaporation also contributes slightly, removing energy (heat) from our skin, cooling us faster.
Extremely low wind chill temperatures are becoming rarer
It is a fact that due to further warming, extremely low wind chill temperatures are becoming rarer (figure 2). Looking at the trend (thick line), the lowest wind chill temperature per winter in over half a century has risen about 6 degrees Celsius. A wind chill of -15 degrees or lower (yellow code guideline) occurred almost every year in 1950, now only once every 5 years. Extreme penetrating cold with wind chill below -20 degrees (orange code guideline) lasting at least an hour has always been rare. In 1950 it occurred once every 5 years, now once every 30 years.
Exact predictions about future trends cannot be made, but KNMI’23 climate scenarios show that while average winter wind speed in De Bilt remains nearly constant, the average winter minimum temperature will rise between 0.7 and 1.4 degrees Celsius by 2050 and between 0.7 and 4.1 degrees Celsius by 2100 compared to the 1991-2020 reference period. Assuming constant wind speed, wind chill temperatures in winter will thus increase.
The coldest winter day in De Bilt will warm by 1.2 to 2 degrees Celsius by 2050. However, this does not mean icy days will no longer occur, requiring KNMI warnings. Extreme values remain possible, even in a future climate. Last winter and early this month, wind chill temperatures around –15 degrees Celsius were measured in the far northeast of the country. During the very cold period in February 2012, wind chill temperatures below –28 degrees Celsius were recorded.
In a future with fewer low temperatures, we may become less accustomed, making warnings for such wind chill temperatures potentially more necessary. Then, the limits of our body are tested, showing how treacherous colder periods in milder winters can be.