The heat wave engulfing northern parts of India can lead to serious health issues, including heat exhaustion and heat stroke. It is important to remember that beyond just the high air temperature, wind, humidity and sun can also play a role. These other factors affect how hot it feels to a person, which can vary quite a bit from the air temperature. Known as the Heat Index, how hot it feels isn't just a matter of mental comfort, it also affects the body's response to the heat.
A strong dry wind can remove moisture from the skin very quickly, fooling a person into thinking that it is not so hot because there is no sweat.
Conversely, high humidity with relatively less heat can also be dangerous without the cooling effect of evaporating sweat.
Finally, direct exposure to sunlight can increase the heat effect by up to 8ºC above the air temperature.
The chart below shows the Heat Index, a way to indicate how the combination of air temperature and humidity can lead to a perceived temperature. For example, if the air temperature is 40ºC and the relative humidity is 25%, it can feel to a person as if it is 41.1ºC, which is not very different. However, if the humidity is 35% then it will feel as if the surrounding temperature is 45.6ºC. As another example, if the air temperature is 45ºC and the humidity is 25%, it can feel to a person as if it is 50.6ºC.
To find the Heat Index, along the left side locate the closest temperature value to the outside temperature, then find the relative humidity that is closes to the actual condition along the top. Where the temperature row and humidity columns meet is the Heat Index. The Indian Meteorological Department provides temperature data, but not relative humidity, unfortunately. Other weather services, such as Yahoo, do provide the humidity.
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