“Humidex” anyone?


Well, it’s been stinking hot across the entire region this week. Oppressive temperatures outdoors – which may be even worse on upper stories of un-cooled buildings. When overnight lows don’t bring much relief this can quickly feel overwhelming.

To be sure, there are different kinds of hot, including the oft-heard line: “It may be x degrees, but it’s OK because it’s a dry heat.” No such luck here, though, where humidity bogs things up even further. With high humidity the body’s air conditioning system can’t do its job: buckets of sweat produce little real relief.

Thankfully, passing thunderstorms have occasionally broken that swelter.

The effect of humidity is so noticeable that Canada invented what’s called a “humidex” (humidity index) back in 1965. In lay terms, this combines the heat of the current air temperature with the humidity of the dew point – does that help? Or would you rather just hear what it “feels like?”

Here’s a more explicit explanation/worksheet on what the humidex is and how it’s calculated.

The formula goes like this:  Humidex = T + (0.5555 * (e – 10)), where T is the temperature in Celsius and e is the vapor pressure in millibars (mb).

Of course, that needs further conversion into Fahrenheit for U.S. consumption. The U.S. prefers something called the heat index, which Wikipedia says produces a resulting “felt air temperature” or “apparent temperature.”

Environment Canada senior climatologist David Phillips spoke with CBC news about the popularity of the humidex:

“It may be impure from a scientific point of view, but it is actually proven to work. There is more stress on the body in a humid situation. You can’t be as productive on a humid, moist day as you would be on a hot, dry day.”

Humidity is but one consideration when it comes to comfort. There’s also sun intensity, often measured with a UV index. The UV index runs from a low of 0 to a high of 15. In Ottawa at least, 9-10 seems to be about as high as that gets. I grew up in Hawaii where the UV index was 12 the day I wrote this, which is pretty typical.

So you’ll get a worse sunburn – faster – closer to the equator. But if you want to talk steamy, drippy, “please-let-it-end!” discomfort…Ottawa heat waves beat Hawaii – hands down.

Photo: Mr. T in DC, via creative commons

Photo: Mr. T in DC, via creative commons

Newscasts certainly seem to be devoting more time and importance to weather. Taking the cynical view, it’s a cheap way to repetitiously fill time with a safe, popular topic. (Weather is much faster to explain than the European debt crisis and more entertaining than elusive Senate reform.) On the other hand, people really DO care about weather and want lots of details.

Some are critical of “feels-like” measures such as the humidex and even the better-known wind chill factor. As with this site, where physicist Miguel Tremblay explores the topic at length, including doubts about applying science to quantify a “feeling,” the rise of sensationalism in meterology and the use of “debatable assumptions” in some of those formulas.

What do you think? Has weather coverage just gone overboard? Do we need things like a humidex? Or are you satisfied with the basics: highs and lows, wind speed and direction and likelihood of rain?

Lastly, what’s your idea of a perfect temperature? Mine is probably a high of 26 C / 78 F and an overnight low of 16 C/ 60 F. What Canadians contentedly call “good sleeping weather.”

Stay cool and take comfort in the old adage “this too shall pass.”

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1 Comment on ““Humidex” anyone?”

  1. If Clapton is God, Warren Haynes is Jesus says:

    Come on, I’m trying to figure out how to short the European housing market given the impending collapse of the Euro. I have little time to pay attention to the heat index, relative humidity vs. the percentage of humidity, UV index, etc.

    And how about the Senate approving three Obama administration nominations in one week? The horror!

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