Yeah i know what u mean, and since the article is about italie which uses celcius its even weirder :)

So the proper conversion from Celsius to Fahrenheit is F = 9/5C + 32

But an easy way to do it (roughly) from Celsius to Fahrenheit is double it and add 30.

To convert from Fahrenheit to Celsius is subtract 30 then halve it.

One helpful tip I've figured out. 100 foreignheight is body temperature. So if it's around 100°F it's around 38°C.

I think about the difference between the two using differences instead of absolutes. That looks like this:

It's kind of hard to do this calc:

F = [ (9/5) * C ] + 32

Or this one:

C = (5/9) * (F - 32)

I refer to those as absolute equations. You have to take into account the pesky offset everytime you want to convert. What if we drop it? This makes:

F = (9/5) * C = 1.8 * C

C = (5/9) * F ~= 0.6 * F

I refer to those as relative or difference equations because if you subtract a temperature from the other, you get the same thing:

F1 = [ (9/5) * C1 ] + 32

F2 = [ (9/5) * C2 ] + 32

F2 - F1 = [ (9/5) * C2 ] + 32 - { [ (9/5) * C1 ] + 32 }

= [ (9/5) * C2 ] - [ (9/5) * C1 ] + 32 - 32

= [ (9/5) * C2 ] - [ (9/5) * C1 ]

= (9/5) [ C2 - C1 ]

F2 - F1 = (9/5) (C2 - C1)

∆F = (9/5) ∆C

So, why is this useful?

Say you have a temperature in Celsius and want to go to Fahrenheit. Simply multiply that number in your head by 1.8 (or think of this as multiplying by 180° as in trig) and finally add to 32. So, 1 °C is (1 * 1.8) + 32 °F or about 34 °F.

Going the other way is a little bit weirder. I make approximations when going the other way by thinking of 180° and how that can be divided. So, 180°, 90°, 45°, etc. corresponds to 1.8 °F (1 °C), 0.9 (0.5 °C), 0.45 °F (0.25 °C), etc. I also approximate by choosing the nearest multiple of 5 or 10 °C (9 or 18 °F). So, 44 °F is between 41 °F (5 °C) and 50 °F (10 °C), closer to 41. It's off by 3, which is about 3.6, which is 2 in Celsius world. This means 44 °F is about 7 °C.

Hope you get the gist! Celsius really is better. I remember this in a pinch:

10 °C = 50 °F

20 °C = 68 °F

30 °C = 86 °F

40 °C = 104 °F

50 °C = 122 °F

Etc.

The freezing temps are a little hard since you cross zero into negatives, but the extrapolation can help

A little helper to get a feel for it: 20°C is 68°F, 25°C is 77°F, 30°C is 86°F etc. Meaning, you just remember one or two combinations and then for every 5°C change it's 9°F up or down