What is the humidex?

The humidex is, as its name indicates, an index, i.e. a calculation. Taking into account the temperature and the humidity, its goal is to measure the comfort, the temperature felt by a human. Most often used from a temperature of 30 degrees, the humidex is in direct link with the wind chill which makes the temperature felt vary. Its value is however not a felt temperature, but an index.

Why was the humidex created?

And who created it? The humidex (or just the humidex) was invented in Canada in 1979. More precisely, it was J. M. Masterton and F. A. Richardson first developed the humidex formula at Environment Canada's Atmospheric Environment Service (now the Meteorological Service of Canada).

Heat and humidity (water vapour content in the air) do not mix well (the hotter and more humid it is, the less comfortable it is), which is why this index measures this relationship. As you may know, it can be very cold in Canada… but it can also be very hot! The country is well acquainted with extremes.

Measuring only heat or only humidity is not enough to give a clear picture of how heat feels to a human being. You have to combine the two and that is exactly what the humidex does.

When is the humidex used?

Meteorologists in Canada are still the main users of the humidex today. In the United States, they use the heat index, which is based on relative humidity instead of the dew point as the humidex does.

The humidex is used to measure the perceived temperature taking into account the humidity. This is a value that only really matters when the temperature is above 30 degrees when it starts to get hot.

In concrete terms, the humidex varies according to the temperature under cover and the relative humidity.
The humidex is also referred to as the comfort index, but it is not just comfort in the sense of pleasure. It is rather comfort of healthy survival! Because when this "comfort" decreases (when the index increases), your health suffers the consequences…

Knowing the humidex, therefore, gives you an indication of how comfortable it is outside: should you go outside? Better still, can you go out without risking your health?

Measuring temperature, heat, humidity, air quality and all other weather data is easy to do from home! The Netatmo Smart Weather Station gives you all this information (and more) on your smartphone. If you are determined, you can then calculate the humidex based on the formulas below!

How is the humidex calculated?

Like any index, the humidex has its calculation formula, its equation. The original comes from Canada, but there are variations elsewhere.
The humidex is often calculated either from the dew point (most often) or from the relative humidity. It all depends on the known values.

Calculating the humidex from the relative humidity

If the relative humidity is known, the humidex is calculated as follows:
Humidex = Ta + h
h = 5/9; (e- 10.0)
e = 6.112 ; 10 ( 7.5;Ta / (237.7+Ta) ) ; HR/100
Where Ta = air temperature (°C), RH = relative humidity (%) and e = vapour pressure.
Calculating the humidex from the dew point
On the other hand, if the dew point is known, it is calculated as follows:
Humidex = Ta + h
h = 5/9 ; (e - 10.0)
e = 6.11 ; exp (5417.7530 ; ( (1/273.16) - (1/Td) ))
Where Ta = air temperature (°C), Td = dew point temperature (°K), e = vapour pressure and exp = exponential function in natural base = 2.71828182845905.
Another way of presenting the equation is as follows:
where Tair is the air temperature (degrees Celsius), e = 2.71828 and Trosee is the dew point (degrees Celsius).

Interpreting the humidex correctly

The humidex is calculated on a scale that ranges from "less than 15" to "more than 54". Note that up to 30, heat and humidity do not affect comfort. However, between 30 and 45, the atmosphere becomes increasingly unbreathable. Breathing becomes difficult and the heaviness of the air unbearable. When the humidex exceeds 45, there is a real danger and above 54, heatstroke is a real danger to life.

In fact, as the humidex increases, so does the feeling of discomfort… until it leads to physical discomfort. The human body reacts badly to too much heat combined with very humid air.
When heat is combined with too much humidity, drinking plenty of water is no longer enough and the discomfort quickly becomes a life-threatening situation.

In your Netatmo Smart Weather Station settings, you can choose to base the temperature felt either on the humidex or on the heat index. However, be careful not to confuse the humidex value with any kind of perceived temperature! 35 does not mean that the temperature felt is 35 degrees. The index is only used to determine where the above-mentioned guide values lie.

The limits of the humidex

The humidex is useful for measuring the comfort and danger of a certain level of heat combined with high humidity, but it is far from perfect. The main criticism is that it does not take into account the skin's exposure to the sun (which increases its temperature to the air) or the wind, which contributes to the evaporation of sweat and therefore to the cooling of the skin.

Some elements are thus missing for the humidex to determine whether the external environment is viable or unbearable. However, there is another, more comprehensive index: the Wet Bulb Globe Temperature (WBGT) index. This index takes the understanding of the actual temperature felt a step further by taking into account the effects of temperature and heat and humidity while including the effects of solar radiation on humans.

A temperature of 35 degrees with dry air is not felt in the same way as a temperature of 35 degrees with humid air. The heat is higher, the comfort lower and the discomfort greater. The humidex helps you better understand how your body reacts to the climate by effectively integrating heat and humidity. You will no longer be able to say that you did not know!