Monday, April 18, 2011

Heat Index

  The other day, after 6 hours of continuous work on an insane project, I was absolutely drained. I was working in CFI, cursing for the first time that Sunday was a holiday, cause if it weren't, labs in my department would have been open, and I would have been working at 21*C, <10% humid environment instead of 37*C and somewhere around 90% humidity!! After I came back to room, I had a bath, a 500ml of Grapo fizz, and 1L of glucose, and yet was feeling bad, dehydrated, and sort of feverish. This made me wonder, was it really 37*C, cause 37*C on any other day, I could have taken it with pleasure, oh not really but still wouldn't have been this bad, afterall its my body temperature on any normal day. A little googling gave me the answers.

 And the answer is the Heat Index. As I am writing, Chennai's thermometric temperature is 86*F or 30*C but the apparent temperature is 99*F or 37.222*C. So why is it so??!! Here's the answer:

  Our body loses excess heat in three ways — radiation, convection and evaporation. If the blood circulating just beneath the skin is at a higher temperature than the surrounding air, heat is lost through the first two ways. When the outside temperature is higher than the body temperature, evaporation is the body's way of losing heat. We start sweating and the evaporation from the skin keeps us cool. The problem is that the capacity of air to hold water vapour is not finite; it can only hold a particular amount at a particular temperature. The term used to indicate how much of that capacity has been used up is relative humidity — the ratio of vapour contained to the maximum the air can hold, expressed as a percentage. So, the higher the relative humidity, the lower will be the capacity of air to hold any more water vapour. That means evaporation slows down and hence its cooling effect too is slowed down, making us feel hotter.

Fig: A screen shot from Weatther underground. Click on pic to expand.

Courtesy: Google, Times of India & Weather Underground

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