This page shows two of the most interesting tables (Humidex Index and Thom's Index) useful to evaluate how current temperature and relative humidity can affect the sultriness or discomfort sensation and cause health danger in the population.
During summer season, or in very hot periods anyway, the human organism makes use of perspiration to maintain its temperature within proper physiological limits. The sweat, which is most made of water, evaporates (endothermal phenomenum) taking away heat so as to have a cooling effect on the skin. An high humidity level in the surrounding environment may obstruct this process limiting the evaporation. The human body so can't eliminate the excessive heat (compared to its own physiological limits) receiving a sensation of an higher temperature.
Obviously all the formulas used to calculate the discomfort indexes produce theoretic average data which may be highly influenced by a lot of human and environmental factors such as: height, weight and sex of the individual, clothes used, presence of shade or wind, carried out activity, etc.
For graphical reasons in the tables you can find combinations of temperature (degrees Celsius) and relative humidity which can't really occur as they exceed the ability of our atmosphere to hold water vapour (e.g. temperature 38 C° and 90% relative humidity)
HEAT AND DISCOMFORT INDEX
HUMIDEX INDEX OF APPARENT TEMPERATURE ( degree C )
| Up to 29 C° | No discomfort |
| From 30 to 34 C° | Slight discomfort sensation |
| From 35 to 39 C° | Strong discomfort. Caution: limit the heaviest physical activities |
| From 40 to 45 C° | Strong indisposition sensation. Danger: avoid efforts |
| From 46 to 53 C° | Serious danger: stop all physical activities |
| Over 54 C° | Death danger: imminent heatstroke |
| Up to 21 | No discomfort |
| From 21 to 24 | Less than half population feels discomfort |
| From 25 to 27 | More than half population feels discomfort |
| From 28 to 29 | Most population feels discomfort and deterioration of psychophysical conditions |
| From 30 to 32 | The whole population feels an heavy discomfort |
| Over 32 | Sanitary emergency due to the the very strong discomfort which may cause heatstrokes |