The Humidex

I listen to the radio when I'm in my car. Usually every hour or so I catch the news, and being Canadian, one of the most important features is always the weather. The winter is a whole different ballgame for me, once the temperature drops below a certain point I can't really tell the difference anyway, so I'm resigned to suffering without caring just exactly how much I'm supposed to be suffering.

In the last few years it would seem that the "humidex" plays a larger role in the delivery of the summertime weather presentation. The presenter will say "It's 29 degrees outside, but with the humidex it feels like 36." I always thought this was silly, because I don't know what 36 feels like. Just yesterday it was 36, but they told me that with the humidex it felt like 43. How the heck am I supposed to know what anything feels like because they are always telling me it feels like something else.

I decided to get to the bottom of this phenomenon, and this is what I discovered;

Now, I hope this clears up a lot of the confusion as to how they arrive at the "humidex" readings. It's pretty straightforward, no? Well then maybe the Wikipedia explanation will be a little simpler;

The current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the heat index used in the United States in being derived from dew point rather than relative humidity.

The record humidex in Canada occurred on July 25, 2007, when Carman, Manitoba hit 53.0.[1] This breaks the previous record of 52.1 set in 1953 in Windsor, Ontario (The residents of Windsor would not have known this at the time, since the humidex had yet to be invented).

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). If the temperature remains 30 °C and the dew point rises to 25 °C (77 °F), the humidex rises to 42. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.

The humidex formula is as follows:

humidex = (air temperature in Celsius) + h
h = (0.5555)*(e - 10.0)
e = 6.11 * exp [5417.7530 * ((1/273.16) - (1/dewpoint in kelvins))]

I suppose I will never really know what those numbers mean. I will just know that it's hot. That's all they really had to tell me anyway.