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Calculating the humidex

June 20, 2012 @ 16:45 By: gordon Category: Weather

As almost everyone certainly is aware by now, it’s hot in Ottawa today. The “low” this morning was about 20°C and it has hit 32°C for a high.

However, it feels much hotter than that because of the humidity. This perceived temperature is known as Humidex. Environment Canada reports the humidex whenever the temperature is at least 20°C and the dew point (the temperature at which the moisture in the air will condense) is above 0°C and when the humidex is at least 25. So, on hot muggy days, you’re probably going to see a humidex value in the weather reports, which you can see in this graph that shows the air pressure (blue), temperature (red) and humidex (orange) for the last 24 hours in Ottawa:


You can calculate the humidex yourself using the formula found on Environment Canada’s website, which I’ve adapted slightly for clarity:

a=6.11 * e^{(5417.7530 * (\frac{1}{273.16} - \frac{1}{(dewpoint+273.16)}))}

b=(0.5555) * (a-10.0)

Humidex=temperature + b

The “e” in the first equation is a mathematical constant that is approximately equal to 2.7182818284.

So, if the temperature is 32°C and the dewpoint is 23°C, the humidex will be 42 when you plug the numbers into the equations and round up. According to the Canadian Centre for Occupational Health and Safety, a humidex of 42 will cause “great discomfort” and you should “avoid exertion”.

Of course, it’s the Wednesday before the Ottawa Dragon Boat Race Festival, which means I have two dragonboat practices this evening. At least I’m steering rather than paddling! If we’re lucky, maybe the 30% probabiliy of thunderstorms will transpire and knock some of the humidity out of the air:

TAF CYOW 202038Z 2021/2118 22007KT P6SM SCT050
PROB30 2022/2024 VRB15G25KT 2SM TSRA BR BKN015 OVC040CB
FM210000 26005KT P6SM FEW040
BECMG 2114/2116 22012KT
RMK NXT FCST BY 210000Z=

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