The humidex explained

Some people have found a way to beat the heat. Peter Power/Toronto Star via Getty Images

You walk out the door today — a day that is already at 30°C at 10 a.m. — and within a minute or two, you can already feel the sweat breaking out across your skin.

What you’re experiencing is the effect of humidity. Our bodies attempt to maintain an internal temperature of 37°C. In order to do this, it sweats. Normally the body cools itself by opening the pores on the skin. But if there’s already a lot of water in the air, our sweat can’t evaporate as quickly.

Humidity can make it feel like it’s a lot hotter out than it actually is. And that’s where Environment Canada issues humidex advisories.

Humidex — short for “humidity index,” a value that is calculated rather than measured — was first used in 1965. It was designed to describe how hot or humid weather feels to the average person. It combines temperature and dew point into one computed value. The dew point is the temperature at which air must be cooled for water vapour to condense and form dew or fog. The higher the dewpoint, the more moisture is in the air.

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Read: Why it’s so hot

If the temperature is 30°C but feels like 40°C, that means that the sensation of heat at 30°C with humid air is almost the same as 40°C when dry.

Environment Canada has a rating system:

A humidex range of 20°C to 29°C will be comfortable; from 30°C to 39°C, there will be some discomfort; from 40°C to 45°C people will experience great discomfort and it is best to avoid exertion; anything above 45°C is dangerous and heat stroke is possible.

The humidex that we use now was developed by J.M. Masterton and F.A. Richardson at the Atmospheric Environment Service (what is now the Meteorological Services of Canada) of Environment Canada in 1979.

Americans use the heat index, which uses the temperature and relative humidity rather than the temperature and dew point. Relative humidity is the amount of water vapour in the air represented by a percentage. A simple way of understanding it would be to say that it reflects how much water vapour is in the air compared to what that parcel of air could actually hold at that temperature.

So on hot and humid days like today, it’s best to try to remain indoors in an air conditioned place, if possible. If it’s not, remember to drink plenty of water and not exert yourself.

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