30 is hot.
20 is nice.
10 is cool.
0 is ice.

40 and 50 can just not, please.

As a Norwegian:

• 100 is boiling
• 40 is we all gonna die
• 30 is hot
• 20 is a little warm
• 10 is a little cool
• 0 is cold
• -5 is maybe time for a jacket
• -10 shit, it's freezingly cold outside!
• -15 I'll stay indoors if I can

I regularly convert between the two just by remembering the conversions for 10, 20, 30, and 40. It's actually pretty easy.

• 0C is 32F (of course)
• 10C is 50F
• 20C is 68F (a cool room temp)
• 30C is 86F (reciprocal of 20)
• 40C is 104F

If you ever forget what one of them is, then just add 18F for every 10C from the last one you remember.

This is actually great, I've never found a good way to remember Celsius temperatures. I might go closer to Terrasque's scale though, 30 is definitely hot where I am.

Metric:

10 mm = 1cm, 100 cm = 1m, 1000 mm = 1m, 1000m = 1 km.

1 cm3 water = 1 gram

1 Watt heats 1 gram of water 1 C°

1 dm3 water = liter = 1 kg

1 m3 = 1000 kg = 1 tonne

Imperial:

1 mile = ?? yards = ?? feet = ?? inches

1 ton = ?? stone = ??punds = ?? oz = ?? grain

1 Galon = ?? pints = ?? fluid ounce

1 inch3 = ?? grain = ?? power to heat ?? fahrenheit

There is no system to any of these, they are unscientific and impractical.

How does Imperial still have any relevance as a measurement system?

O F is the freezing temperature of a saturated brine solution, while 100 F was the body temperature of a human. Yes, body temperature has been revised a bit, but the two points were chosen as stable points that anyone could access that would generally be unchanged by pressure changes, etc. Human homeostasis is quite good at keeping a temperature in a narrow range. Also, boiling is massively affected by air pressure. At 5000' elevation, boiling is approximately 202 F and continues to get lower as altitude increases. Lots of people live at higher altitudes. (Hi! I am one of them !)

Edit: I was a little off on the temperature selected for body temp, but still pretty close: https://en.m.wikipedia.org/wiki/Fahrenheit

It is convenient because they are used to it. That is all there is to it, and peace be to that.

It only becomes silly when they begin to claim that F is better for "human temperature", because again it all comes down to what you are used to and celsius is just as convenient if you are used to that.