What Do You Mean, “Humidex”? An Explainer | Vol. 3 / No. 38.4

Photo: Ryan Dickey, CC BY 2.0 

Why how warm it “feels” changes when you cross the border.


So it’s been hot, not just here, but everywhere. In fact, June was the hottest June on record. May was the hottest May on record. April? You guessed it: the hottest April on record. The last six months, according to NASA, have been the hottest six months we’ve ever recorded. And it’s going to stay hot.

This week in Boston it’s been pretty bad during the day, but not awful.

From weather.com

For those of you in Canada — and there’s a reason I’m explaining this — those highs would read 31.7°C, 34.4°C, 32.8°C, 30°C, and back to 31.7°C for two days.

But I can tell you right now that Tuesday of next week is going to feel pretty oppressive by comparison with today, thanks to those little percentages on the right. Those are relative humidity numbers.

And the funnier bit is, it’ll feel differently depending on whether you’re a Canadian or an American. Well, sort of.

Here’s how it works.

Air can hold a certain amount of water, and that amount varies by the temperature and pressure of the air. Assuming the pressure stays constant, the warmer the air is, the more water it can hold. But relative humidity, as you can see in the video below, isn’t a great measure of how humid it actually feels, because the amount of water in the air rises and falls precipitously with temperature.

There are actually two numbers we’re concerned with when we talk about how warm it feels: relative humidity and dewpoint. Dewpoint is the temperature at which relative humidity (the amount of water the air can hold) reaches 100% capacity. The higher the dewpoint, the more sticky and gross (and warm) you’re going to feel.

Now, above, you can see that while today’s 89°F has a humidity of 37%, Tuesday’s 89°F has a humidity of 60%. Using this handy-dandy calculator we can determine that (roughly, all other things being equal), today’s dewpoint is only 59°F, but Tuesday’s is going to be 73°F.

When you see the little box on your forecast that says “feels like” and a number, that’s what they’re taking into account.

Screenshot 2016-07-21 at 9.23.14 AM - Edited

But here’s the funny thing: in Canada and the US we use different formulas to determine what it feels like.

In the US, the formula is called the Heat Index while in Canada, it’s called the Humidex. And as Susan Kieffer writes on her blog Geology in Motion, it’s “amazing” that the numbers the two formulas spit out “are even close to each other because of the difference in form.”

According to Wikipedia, this is the formula for the Humidex:

{\text{Humidex}}=T_{\text{air}}+0.5555\left[6.11e^{5417.7530\left({\frac {1}{273.16}}-{\frac {1}{T_{\text{dew}}}}\right)}-10\right]


  • \scriptstyle T_{\text{air}} is the air temperature in °C
  •  \scriptstyle T_{\text{dew}} is the dewpoint in K

The humidity adjustment effectively amounts to one Fahrenheit degree for every millibar by which the partial pressure of water in the atmosphere exceeds 10 millibars (10 hPa).

And again, according to Wikipedia, this is the formula for the Heat Index: 

\mathrm{HI} = c_1 + c_2 T + c_3 R + c_4 T R + c_5 T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \,


\mathrm{HI}\,\! = heat index (in degrees Fahrenheit)
T\,\! = ambient dry-bulb temperature (in degrees Fahrenheit)
R\,\! = relative humidity (percentage value between 0 and 100)
And the Cs are some possibly contentious constants.

In the end, what these reliances on constants create is a narrow range in which these equations are totally accurate. On top of that the Humidex “baseline” is 7°C (45°F) and the Heat Index is about 14°C (57°F), so at higher temperatures, the Humidex figure is going to say it feels a few degrees warmer.

All of which is to say that if you use the Canadian Humidex, it’ll feel like a slightly warmer day than if you use the Heat Index — at least when the temperatures really get up there.

Of course, these numbers all rely on the idea that it’s not windy and you’re in the shade, which, if you’re in the real world and out doing things, are probably unreliable figures. Plus, if you’re prone to sweating it might feel cooler, if you’re more overweight it might feel warmer — there’s a lot of variable in how warm it feels to any one person at any one time.

So mostly this post is just in good fun: I like to point out the differences between my home country and my current place of abode, especially when it’s a difference that isn’t a completely horrifying social problem not very present north of the border.

So there’s your little science lesson for the day. And remember, when it’s hot out, drink plenty of water, wear sunscreen, and stay in the shade when possible. Because heat stroke and cancer are bad for you.


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Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.