Ah, finally a cool day to node this!
What is it?
The humidex is the Canadian way to describe what the temperature feels like when it’s very hot and very humid. For example, the air temperature on a hot summer day might be 30°C (86°F). But if it’s muggy, and the relative humidity is, say, 80%, the temperature is going to feel more like 43°C (110°F), according to the humidex. Yikes!
What’s happening is that perspiration on the body is doesn’t evaporate as quickly as it does on drier days, since there’s already so much humidity in the air, and so most people perceive the temperature as being hotter.
Other countries call this the “heat index”, which brings us to the next question:
Why is it called humidex?
The name is sort of a meld of the words “humidity” and “index”. Sometimes Canadian governmental agencies will invent their own words for things to make translation easier between the two official languages. The wind chill index is sometimes called the “chilldex”. I’m not sure why the wind chill factor doesn’t cancel the humidex out, or at least lower it a bit, if there’s a bit of a breeze on a sticky summer day, but it never seems to.
What is its history?
The humidex was introduced in 1965 by what is now the Meteorological Service of Canada. The formula currently used to determine the humidex has been in use since 1979.
When should I start worrying?
The Weather Service makes the following assessment of how unpleasant to how dire humidex ratings can seem:
Less than 29°C (85°F): no discomfort
30°C to 39°C (86°F to 103°F): some discomfort
40°C to 45°C (104°F to 113°F): great discomfort; avoid exertion
Above 45°C (113°F): dangerous
Above 54°C (129°F): heat stroke imminent
When the humidex is high enough, the Weather Service of Environment Canada will issue an advisory.
Does it ever really get hot enough in Canada for this information to be useful?
Pretty well the only places in Canada where it gets hot and humid enough for the humidex to play a significant factor for the Weather Service to actually make public service warnings is in the southern parts of Manitoba, Ontario, and Quebec. In terms of cities, this means Winnipeg, Toronto, Ottawa, and Montreal. While geographically this is a fairly small percentage of Canada, in terms of population it's probably two-thirds to three-quarters.
In 2001, Saskatchewan was issued its first humidex advisory in 30 years. Windsor, situated just south of Detroit, is Canada’s most humid city and as of 1995, holds the record for highest humidex at 52.1, on June 20, 1953.
How do I calculate it?
The formula is not for the faint of heart -- those who are faint of heart should be indoors where there’s air conditioning sipping iced tea when it’s so humid anyway. It uses the dew point temperature and is concerns itself with the molecular weight of water, among other things. Fortunately there is a simplified formula:
5*((6.112*10(7.5*T/(237.7+T))*H)-10)
Humidex = T + --------------------------------
9
where H is the percentage of humidity in the air and T is the temperature in degrees Celsius.
Now, calculate away, and remember to keep yourself well hydrated.
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Environment Canada. “Humidity,” Meteorological Service of Canada. http://www.msc-smc.ec.gc.ca/cd/humidity_e.cfm. August 26, 2002.
Anton Skorucak. Question: Humidex Formula, PhysLink.com. http://www.physlink.com/Education/AskExperts/ae287.cfm. August 26, 2002.
Greenpeace Canada. “Weird Weather,” Greenpeace.ca. http://www.greenpeace.ca/e/feature/wweather.html. August 26, 2002.
Natural Resources Canada. “Superlatives: Weather”, The Atlas of Canada. http://atlas.gc.ca/site/english/facts/superweather.html. August 26, 2002.
Canadian Centre for Occupational Health and Safety. “Humidex Rating and Work”, OSH Answers. http://www.ccohs.ca/oshanswers/phys_agents/humidex.html. August 26, 2002.