For the same temperature, the heat will be more difficult to bear in Biarritz with 80% humidity than in Toulouse with 40% humidity. How to explain this feeling of suffocation and discomfort?
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[EN VIDÉO] Kézako: what effect does the wind have on the feeling of heat? If a little refreshing breeze is welcome in summer, we would do well without it in winter. The wind seems to have the gift of making the thermometer lie. But the feeling of heat or cold is in fact a skin-deep experience, which Unisciel and the University of Lille 1 intend to explain in this episode of Kézako.
Newsletters weather report inform us daily about the temperature, but rarely mention the humidity level. However, the latter, which measures the ratio of the quantity of water vapor contained in theair on the maximum amount of water vapor possible, is yet a key factor that makes the heat more or less bearable. Thus, a relative humidity of 70% means that the air contains 70% of the maximum water vapor that it can absorb at this temperature.
Moisture prevents sweat from evaporating
The body is cooled largely by the sweat where heat is removed by evaporation. However, the more humid it is, the less the air can receive additional humidity. In high heat, the sweat no longer manages to evaporate and the body no longer cools. From 90% relative humidity, the air is so saturated that evaporation becomes almost impossible. In addition, sweat tends to “stick” to the skin, which further increases the feeling of heat. So if you can survive temperatures of 50°C in dry air, the heat becomes deadly from 35°C when the air is saturated with moisture. In the absence of cooling, the body temperature can then rise to 42 or 43°C and lead to lethal hyperthermia.
In strong heat, the phenomenon is aggravated because warm air contains more humidity than cold air. At 30°C, air can contain up to 30 g of water per cubic meter in the form of vapour, i.e. 10 times more than at -5°C. Therefore, for the same relative humidity, we feel its effects more at 30°C than when the air is cool (not to mention that we sweat less in cool weather).
Hot air is harder to breathe
Damp air not only prevents the body from cooling, but it is also more difficult to breathe, which increases the feeling of suffocation. This is because the water vapor pushes out a small part of the oxygen in the air. Our body must therefore make an extra effort to suck in the same amount of oxygen.
However, other factors must be taken into account. The wind can thus favor the evacuation of heat and facilitate the cooling of the body. ” During the June 2019 heat wave, the humidity in Paris was up to 80%, but there was a lot of wind, which made the temperature more bearable », explain to New Obs Steven Testelin, forecaster at Météo France.
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