The so-called perceived heat (or apparent temperature) measures the "hot" sensation felt by a subject.
This sensation does not depend on simple external temperature (ambient heat), but also on other exogenous factors - such as relative humidity, clothing, the degree of exposure to ultraviolet radiation and wind speed - and from endogenous factors, such as mass and the individual's body composition or physical activity at a given time.
In order to establish the perceived heat, some mathematical formulas have been proposed that evaluate in an integrated way the environmental temperature, the degree of humidity, the level of radiant energy etc. One of the most famous is the so-called heat index, which is limited to taking in consideration of humidity and environmental temperature.
If the environmental humidity is high, the perceived heat is greater, since it creates a sort of barrier at the skin level that opposes the loss of heat by evaporation of sweat. Conversely, when ventilation is high and humidity is poor, evaporation of sweat is favored (surface vapor pressure is reduced), and convection dispersion is increased (because fresh air flows constantly replace the air that has been heated in contact with the skin).
Other indices used for the calculation of the perceived heat also take into consideration the level of radiant energy, ie the exposure of the organism to ultraviolet radiation (it is clearly different to stay in the sun or in the shade, or wear dark or light clothing).
Looking at the table to the side, which simply considers the effect of the ambient temperature and relative humidity to establish the perceived heat, it can be noted that, for example, at a temperature of only 30 degrees with humidity of 90% the perceived heat is 45 ° C, while an ambient temperature of 35 degrees is more pleasant with 40% humidity (heat perceived 37 ° C). The table is taken from en.wikipedia.org (source: US National Oceanic and Atmospheric Administration)
In meteorological language, the term afa refers to a bad weather condition caused by the simultaneous presence of excessive heat, high humidity and no wind, a mix that makes one perceive temperatures much higher than the real ones. High humidity rates (up to 90% and above) and poor ventilation are frequently recorded during the summer in the lower part of the Po Valley.
