It might sound a little odd to hear about the humidex in our west coast weather forecasts, but if you’re from Ontario or Quebec you’ve likely heard the term before. More and more the humidex is being referenced right across the country.
What is the humidex?
Humidex is short for Humidity Index. The Canadian innovation came about in 1965 to try to describe how hot and humid weather actually feels to the average person. Yesterday afternoon in Port Alberni the air temperature was 31°C, but with the humidity it felt like 33°C. In Nanaimo, the temperature at 3pm was 28°C but it felt like 32°C.
Afternoon temperatures at 3pm Wednesday July 27, 2016 and corresponding humidex values show what the weather felt like that afternoon.
Why does the humidex matter?
Humidex is the perceived temperature when there’s a certain amount of moisture in the air, and it tells us how uncomfortable we may feel in those weather conditions. So, if the forecast calls for a humidex of 40, the air temperature might be just 30°C, but with the humidity, it feels like it would if the air was dry with a temperature of 40°C. A high humidex can cause problems with the body’s ability to cool itself.
Here’s the Environment Canada Humidex table, complete with degrees of discomfort:
How do you calculate the Humidex?
The index is based on a calculation of the current air temperature and the dew point. Math enthusiasts, here’s the formula used by Environment Canada:
Humidex = (air temperature) + h
Where
h = (0.5555)*(e - 10.0);
e = vapour pressure in hPa (mbar), given by:
e = 6.11 * exp[5417.7530 * ( (1/273.16) - (1/dewpoint) ) ]
exp = 2.71828
Dewpoint is expressed in kelvins(K) (temperature in K = temperature in °C + 273.16) and 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.
Where is humidex highest?
The humidex is used all over Canada, but the high readings are quite rare throughout most of the country. In general, as the farther north you go, the lower the humidex value.
If you’re in southern Ontrario, Manitoba or Quebec, though, it’s a different story! Hot and humid air masses originating in the Gulf of Mexico or the Caribbean can invade and send the humidex into dangerous territory. The highest recorded humidex in Canada was measured in Windsor, Ontario on June 20, 1953 at 52.1!
