## 15 September 2007

### just noticeable difference of temperature?

Down with the metre and litre, claims that the sizes of metric units are unintuitive: "On the other hand, a degree celsius is too large and imprecise. By contrast, the Fahrenheit scale was custom made to measure weather phenomena: it has human scale."

A celsius degree is 1.8 times the size of a Fahrenheit degree.

Do people who claim this seriously think that they can tell the difference between, say, 61 degrees Fahrenheit and 62.8 degrees Fahrenheit? I don't think they could, and so I would claim that even the Celsius degree is small enough for "everyday" purposes. (In weather forecasts given in Fahrenheit one often refers to, say, the "low seventies" or the "mid-sixties" which seems to imply a resolution of about three or four degrees.) I claim that in both scales, one degree is less than the just noticeable difference for atmospheric temperatures, although I can't quickly find out if the people who study this back me up.

(One thing I do like about using the Fahrenheit scale for weather is that almost all weather I experience is between about 0 degrees and 100 degrees. In some tellings of the story, that's said to be deliberate. There is something weird about negative temperatures, but I'm not quite crazy enough to just start stating all temperatures in Kelvins.)

Aaron said...

Hmmmm! According to Wikipedia, one version of the Fahrenheit story claims that Fahrenheit calibrated the zero on his scale to the lowest outdoor temperature he could measure, and that he did so specifically to avoid negative temperatures in everyday life. There are a lot of versions of the story, though... :P

Anonymous said...

But on the other hand, if you're living in a temperate climate then the most important temperature of all, and the one temperature where small changes make a very noticable difference, is the temperature at which water freezes. Or ice melts. So I find it very natural to have that being the zero on the temperature scale. Of course, it could just be because that's what I am used to. But 32 strikes me as an odd number for such an important role. (Yeah, there is a joke in there. Don't mention it.)

Harald Hanche-Olsen said...

Argh, the previous "anonymous" post was mine. I never post anonymous comments, but my finger slipped and off into the ether it went.

clintp said...

On the other hand, I live in one of those temperate climate and find the extra gradients in Fahrenheit around 32 degrees more useful than celsius.