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Converting Celsius to Fahrenheit (when you don't have a calculator handy)

Map showing current temperatures across British Columbia and Alberta on May 26, 2017 (Photo courtesy: Kristi Gordon, Global News BC, Vancouver)

The United States is among the last bastions in the world who use the old imperial units for measurements -- be it miles instead of kilometers, pounds instead of kilograms, and of course, temperatures in Fahrenheit instead of Celsius.

But Canada and its Metric System way of life is just a few short hours' drive away and American visitors are probably somewhat stumped when they see the bank signs say it's a balmy 20 degrees when the sun is out and folks are in short sleeves.

Most of you might know the official conversion to get to Fahrenheit to Celsius: Multiply the C temp by 1.8, then add 32 degrees. But multiplying by 1.8 on the fly is a bit of a challenge unless you're a super math whiz.

My father had given me a simple estimation trick when I was little: "Just double it and add 30" -- the "doubling" part overestimation counterbalanced by adding only 30 instead of 32. I'll admit it makes it easy, but is it really that accurate?

Turns out, it depends on the season.

Greg Johnson (you know, of SkunkBayWeather.com fame who runs those amazing web cameras in Hansville) and I were chatting about this and in his "numbers nerdiness" (his words) charted out just how effective my estimation was.

It's spot-on accurate when the temperature is 10 Celsius -- it comes out to 50 degrees F no matter if you "fake it" with my trick or do the actual math. But as it gets warmer, my trick's not as accurate. If it's 20C outside, I'll come up with 70F, but it's really 68F. 27C would become 84F, but it's really 81F. And if you're, say, near the southern Arizona border heading into Mexico for the day, a 36C reading would have me thinking it's 102F outside when it's really "only" 97.

Obviously as far as what it feels like outside, I'm still in the ballpark. But if you're a stickler for the numbers, Johnson suggests maybe tweak my idea to add about 27 instead of 30 in the summer. That has a break-even point at 27C = 77F no matter the calculation and thus the errors are reduced when you start rough-converting in warmer weather.

Or another idea if you want the spot-on number, you double Celsius, then subtract 10 percent, then add the 32. So 22 C would double to 44, subtract 10 percent (4.4) to get 39.6 and add 32 to get 71.6 (72) - the correct number.

Next blog sometime: How to convert kilometers an hour to mph -- no, you're not supposed to drive 100 mph on Canadian freeways. (Handy trick: Take first digit (or first two digits, if there's three) and multiply by '6' to get mph. 50 km/h -- 5x6 = 30 mph (actual conversion 31 mph.) For 100 km/h, take 10x6 = 60 mph (actual conversion: 62 mph).

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