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Temperature felt

When we measure the outdoor temperature we are referring to two separate pieces of information: the temperature of the air, also known as the temperature under shelter, and the temperature felt. While the former is an objective measurement, the latter can vary depending on the atmospheric conditions (wind speed, humidity, sunshine, etc.) and each person's body. So how can meteorologists assess the temperature felt, which is so subjective and variable? And why is it beneficial to fine-tune temperature forecasts based on this indicator?

What is temperature felt?

Standard weather forecasts use the temperature of the ambient air as an indicator. This thermal measurement expressed in degrees is taken using a sensor or a thermometer placed under shelter 1.5 m off the ground. Meanwhile, the temperature felt is an indicator of the sensation of cold or warmth that people actually feel.

Developed in the United States before the Second World War, the notion of temperature felt gradually spread worldwide and in France to give an idea of the sensation of warmth or cold that the human body feels when exposed to different conditions. Although it's in fact based on a measurement of the air temperature, it can be very different, either much higher or much lower.

This is because the sensation of cold is not only linked to the outdoor temperature but also temperature differences between the body and its environment. These are called heat flow exchanges. Other factors also influence the temperature felt. For example, the wind is a decisive component: by removing the heat layer around the skin, this causes a temperature loss. The body then needs to rebuild this natural thermal insulation, which lowers the body's temperature and increases the sensation of cold. This phenomenon is called wind chill and explains, for example, why we feel colder when the wind speed is higher.

Other factors that influence the temperature felt include humidity. If it's raining or when the humidity level in the air rises, the body's cooling mechanism through perspiration is less effective. This makes it more difficult to vent excess heat and the temperature felt rises as a result. The sun's rays on the skin also need to be taken into account to assess the sensation of warmth.

To give a realistic idea of the temperatures actually felt, weather forecasts therefore need to be fine-tuned by taking these variables into account: wind blast and speed, humidity level, sunshine, etc. Nevertheless, the temperature felt indicated by meteorologists is only a general indicator and everyone will perceive warmth or cold differently depending on their general state of health, age and body type. Discover all our products

Why do we measure the temperature felt?

Knowing what the temperature felt is allows us to fine-tune the temperature measurements provided in weather forecasts. This allows the most exposed individuals to prepare themselves more effectively. As such, people who work outside or who do sports or leisure activities outdoors can adapt their equipment, clothing or water provisions based on the temperatures forecast.

However, the effect of this perception indicator goes much further. Indeed, Météo France uses it in extreme weather conditions, when the heat or thermal cooling can pose a threat. The wind chill factor is also important in terms of plans put in place to help the homeless when the weather is extremely cold, and the temperature felt can be used to assess the level of monitoring required in each geographical area if there's a heatwave.

That's why, although meteorologists don't always recognise the temperature felt indicator, its use is extremely important for determining the temperatures that people are exposed to. If a thermometer measures an air temperature of -10 degrees the cold is bearable, but if you add a 30 km/h wind to this, the perceived cold can be as much as -20 degrees and pose a risk of hypothermia or frostbite. This is why the temperature measured by a thermometer need to be weighted in order to determine the body's actual sensation.

This is even more important if the weather conditions are extreme, as in Scandinavian countries. Nevertheless, even in France this information is useful in bitterly cold weather conditions or when heat events may pose a threat to the most fragile or exposed people.

How is the temperature felt measured?

There's no universal method for measuring the temperature felt. The air temperature indicated by a thermometer under shelter is obviously used as the starting point and the measurement is adjusted based on an estimate of the wind speed, the risk of gusts of wind, the ambient humidity and the amount of sunshine expected.

There are tables that indicate the wind chill factor based on the air temperature in degrees and the wind speed in km/h. This indicator is a unit-less number, although it is often wrongly expressed in degrees. This confusion is nevertheless understandable, as the indicator expresses a sensation on the skin comparable to an equivalent air temperature. More simply, when we talk about a wind chill factor of -10, the effects on the human body are similar to those felt when the air temperature is -10 degrees.

Meteorologists at the WMO (World Meteorological Organisation) are working on another indicator that provides a more accurate assessment of the bodily sensation, a "universal thermal climate index" that takes into account the exchanges between the air and the human body.

In addition to the outdoor temperature, humidity and various environmental factors (wind speed, precipitation, air quality, etc.), most weather stations indicate the temperature felt. This is an excellent way to get a quick idea of the outdoor conditions so you can take the appropriate clothing or equipment, or adapt your schedule of activities.

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