Lis back! With him, his procession of pleasant images, afternoons at the beach or walks in the mountains. Unfortunately, climate change is now also burdening the summer season with a certain amount of anxiety, the keywords of which are drought, fires and heat waves… The extraordinary heat wave that certain countries like India are going through, where extreme temperatures have been recorded, adds an apocalyptic turn to this anxiety-provoking context. And poses a fundamental question to all of us: will human beings and nature be able to adapt to these extraordinary episodes which specialists predict will multiply and intensify?
From a physiological point of view, human beings are endowed with an incredibly effective thermoregulatory function, sweating. When the skin temperature rises above about 30°C, the sweat glands are stimulated and release sweat. This surface water will evaporate using the thermal energy present in our skin and contribute very significantly to preventing the temperature of our body from exceeding the critical limit of 42.3°C. Beyond that, our proteins become denatured, leading to the disruption of biochemical processes and death.
The cooling mechanism is quite simple and uses the fact that water molecules in the liquid phase are held together by an attractive force of electrostatic origin. The process of evaporation corresponds to their detachment from the film of sweat, under a random fluctuation, and their escape into the atmosphere. By detaching themselves from their congeners, they take energy from the skin and tend to lower the temperature. Thus, even at rest and at 20°C, 1020 water molecules per second!
Atmospheric water content
In principle, the hotter it is, the more efficient the mechanism. However, a problem may arise, because the outflow of water molecules is counteracted by the inflow of water molecules present in the form of vapor in the atmosphere and which constantly bombard our body. When the humidity level is high, this counter-flow considerably reduces the efficiency of transpiration and even cancels it out when the atmosphere is saturated with humidity (which happens in certain regions of the globe). To integrate this moderating effect of the water in the atmosphere, we define the notion of “wet bulb temperature” (TH), which is the lowest temperature that can be obtained in an isolated place by letting evaporate as much water as needed to reach saturation. It is measured by wrapping a damp cloth around a thermometer and fanning it. The TH of water-saturated air is the actual temperature (because there is no longer net evaporation), but it is lower for dry air.
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