You parked your car in the sun for just a short while. Nevertheless, the interior temperature quickly climbed to 65°C or above, enough to melt that favourite music cassette you left on the dashboard. You drive off and the car’s air conditioner slowly becomes effective. The large glass windows, however, can’t prevent the sun’s radiation heating parts of your body. You feel very uncomfortable. On top of that, a spate of road accidents reduced the traffic to a crawl.
An early sign of heat stress is tiredness, exhaustion or irritability. Any physical or mental task becomes stressful and the performance suffers. Tests of workers with different skills established that the accuracy in physical and mental tasks drops quite markedly even at relatively mild temperatures between 28.5°C and 31°C. Temperatures of 32°C and above leads to a notable decrease in short-term memory.
Based on studies, here are four good examples:
- In an audio vigilance test, Morse code operators made significantly more errors with increasing temperatures.
- Results of examinations performed by students in air conditioned classrooms are significantly better than the results of students who completed the same exams in a warm environment.
- Statistical evidence associates a rise in numbers of assault, robbery, domestic violence and rape with high temperatures.
- Body temperatures of military pilots rise by up to 2°C during flight and while exposed to the sun’s radiation under Perspex-canopied cockpits.
In the last example, the study shows that the pilots had to perform with an elevated body temperature equivalent to a fever. Simple tasks are no longer simple. Pilots miss checklist items, flick the wrong switches and violate rules and regulations.
Therefore, it comes as no surprise that the number of road and work-related accidents increase during hot and humid conditions.
We can tolerate hot and dry conditions better than hot and humid conditions. On humid days the air ‘feels’ hotter than it actually is, and vice versa on dry days. This apparent temperature is based on relative humidity and air temperature.
For easy use, the US National Weather Service developed an internationally recognized heat index table. The creators of the index assume that you are in the shade and that there is no wind. For example, an air temperature of 30°C has the effect of 41°C when the relative humidity is at around 90%. In addition, temperatures in direct sunlight are up to 8°C above the temperatures in the shade.