Humidex vs. Heat Index: What's the difference?

Tuesday, July 30, 2013, 11:37 AM -

What is the humidex? Typically when I hear that question the person simply wants to know what it “feels like” outside due to the heat and humidity. However, with the unusually humid weather that we experienced at times earlier this summer in Atlantic Canada as well as across Ontario and Quebec, I think that it is worth revisiting the question to better understand what the humidex is (and is not).

The humidex was developed by Canadian meteorologists and was first used in 1965 to describe the impact of high humidity on human comfort. A warm day will feel even warmer when the humidity is high because our perspiration (sweat) does not evaporate as readily when the humidity is high.

(Environment Canada)

Evaporation is a cooling process that we all experience when we step out of the shower or the pool and rush to grab a towel. Perspiration is our body’s natural attempt to avoid overheating, but when that sweat cannot evaporate as quickly, the heat will have a greater impact on us.

A humidex over 40 is considered to be extreme and Environment Canada recommends that all unnecessary outdoor activity be curtailed under those conditions. When the humidex reaches the mid to upper 30s, rigorous outdoor activity should be reduced with special attention paid to the age and health of the individual.

During the mid-July heat wave in southern and central Ontario, the humidex reached 47 in St. Catharines and mid-40s were reported at numerous other locations. Especially noteworthy was that the humidex did not drop below the mid-30s for several nights in a row at several locations, especially in the Toronto area. 

During July the Maritimes have also seen the humidex top 40 on a few occasions and St. John’s Newfoundland came very close several times to breaking their all-time humidex record of 38.5.

As oppressive as the humidex has been this summer, it has been worse in Canada in the past. On June 20, 1953, Windsor Ontario had a humidex of 52.1 and on July 25, 2007, Carman, Manitoba reached 53.0. 

If you have travelled to the United States or read reports about heat waves in the States, you have likely heard about the Heat Index. It should be noted that the equation used to calculate the humidex is different from what is used for the Heat Index in the United States. Therefore, one cannot convert the Heat Index to Celsius to compare conditions in the two countries. For example, at a temperature of 32 °C with a relative humidity of 50%, the Heat Index would be 35 °C, but the humidex would be 39.

Furthermore, while the Heat Index is reported in °F, the humidex is a unit-less number, meaning that it is incorrect, to report it in degrees Celsius. 

However, the number does roughly correspond with what the air “feels like” in °C, and is often reported that way. 

It should be noted, though, that subjective assumptions went into developing each equations, which is why the same conditions will “feel like” two different numbers depending on whether it is being reported in Canada or the United States. Also, neither index takes into consideration wind speed or cloud cover vs. sunshine which also impacts how a given temperature and humidity will impact humans and pets. 

The last week of July and the first week of August often bring some of the muggiest weather of the summer, but so far the final week of July has brought an autumn preview from Alberta to Quebec. What will August bring? Stay tuned to The Weather Network for our summer update coming next week.