It is just past one o’clock here and the thermometer says zero. That’s Celsius, of course, and it sounds worse than it is. In our more familiar Fahrenheit system the air temperature is a balmy thirty-two degrees on the plus side of zero.

The centi-grade system is based on one hundred degrees as you would expect. Water freezes at zero and boils at one hundred.

Fahrenheit’s scale uses one hundred-eighty degrees for the same span: water freezes at thirty-two and boils at two hundred-twelve.

Both are arbitrary. The thing I like about Fahrenheit is when the temperature climbs into triple digits, from the 90s to the 100s, you know it is really hot. The Celsius scale has you going from the mid 30s to the low 40s and it just isn’t the same. Our body temperature of 98.6º F translates to 37 ºC and so a jump to 100 ºF, which is dramatic, seems less so when it is only a jump to 38 ºC.

Other than that you can get used to the Celsius scale easily enough. I like to remember a couple of key points like room temperature which at 68 ºF is 20 ºC. That’s a good point to start. Every five Celsius degrees is equal to nine Fahrenheit degrees so you can just count by fives on one side and add nine on the other.

25 ºC is (68+9) 77 ºF and 30 ºC is (77+9) 86 ºF and so on. You can go down as well. Remembering that 20 ºC equals 68 ºF means that 15 ºC is (68-9) 59 ºF and 10 ºC is (59-9) 50 ºF and so on.

I suppose that is only handy if you are travelling in England or whatnot and they give the weather forecast in Celsius. By the way if you use my subtraction method above all the way down to -40 on the Celsius side you will find that it will equal -40 on the Fahrenheit side! Just a little quirk in the two systems—they are offset by thirty-two degrees and a Celsius degree is 1.8 (9/5) times bigger than a Fahrenheit degree. If the two temperature scales are plotted on a graph the slopes of the lines will be different and that means they have to intersect somewhere.

Thirty-two degrees Fahrenheit (zero Celsius) isn’t all that inviting for a walk around the block but is pretty mild overall. We can get days that are below freezing, in the teens and 20s, which puts you into negative numbers in Celsius (-5 ºC = 23 ºF and -10 ºC = 14 ºF).

It seems like negative numbers should be reserved for the really cold days and the Fahrenheit scale works like that. Anything below zero Fahrenheit (-18 ºC) is really cold, something on the scale of winters in North Dakota. The coldest temperature we have recorded living here thirty-plus years is 10 ºF (or -12 ºC). People in Nebraska get that kind of stuff all the time which makes me happy I live here and not there.

Speaking of variation, you probably know from experience that water boils at different temperatures depending on the altitude (elevation above sea level). That kind of thing, plus the arbitrary nature of the temperature scales, has scientists in search of a more absolute way to measure temperature. The Kelvin scale is based on the idea of absolute zero, when molecular motion is so insignificant it cannot be measured as heat. This scale zeroes out at about -459 ºF (-273 ºC).

If I convert today’s 32 ºF (that is, 0 ºC) to the Kelvin scale I get, interestingly, 273 Kelvins (you don’t say “degrees Kelvin”, just “Kelvins”). A Kelvin, it turns out, is the same size as a Celsius degree, but the scale starts at a different place, absolute zero.

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