You might already know this. If you don’t then there is something you’ve never noticed before, that you will always notice from now on.
From time to time you will come across a report by someone who has visited one of Earth’s cold places. The Antarctic; The Himalayas; the meat fridge in Tesco. At some point in the report they will say something like “… I checked the thermometer – it’s still minus forty. Brrr ….”
The odds are that they are wrong. It’s quite likely that the temperature is much colder than that.
Why? I’m willing to bet they have a Mercury thermometer, and one of the things that “not a lot of people know” about mercury is that it freezes solid at around -39 degrees Celsius. So when it gets really cold – below minus 40 cold – a Mercury thermometer isn’t any use any more. Once that happens, and if it gets warm again reasonably quickly (not likely in Tesco, but very likely – according to Al Gore – in Antarctica) your thermometer is – to use a technical term – fuxxored, and you might as well throw it away and buy a new one.
By the way, there’s something else peculiar about minus forty: it’s the point at which the Fahrenheit and Celcius scales intersect. So if you know it really is minus forty, you don’t have to say which temperature scale you are using. If it’s minus forty Celsius, it’s also minus forty Fahrenheit.