TLDR;

For the majority of people who will be switching from Fahrenheit to Celsius, this is all you need to know.

Celsius	Action to take
Below 0 °C	It is frozen, wear a jacket and possibly themals. The further below 0 °C the more warm cloths you need to wear.
0 °C to 10 °C	Its cold, but not freezing. Wear a jacket.
10 °C to 20 °C	There is a chill, a shirt is fine if you're into colder weather.
20 °C to 25 °C	The temperature is ideal for human beings.
25 °C to 30 °C	It is warm but not hot.
30 °C to 40 °C	It is hot, unless necessary don't go outside or in the sun.
over 40 °C	These places exist on earth, but I have no idea why anyone would live there.

What is Fahrenheit

Fahrenheit was a scale that (to the best of our knowlege) was derived by 0 °F being a stable solution of ice, water, and ammonium chloride, and the temprature of a health human being 96 °F. In the final standard of the scale the healthy human ended up being 98.6 °F and freezing water at sea level was 32 °F.

In short, this was a scale created in 1724 based on some faulty assumptions. If you would like to read more about it then I suggest you read the Wikipeida Article on Fahrenheit.

What is Celsius

Celsius (also called centigrade) is a scale of temprature that is based on water at sea level. In Celcius, 100 °C is boiling and 0 °C is freezing. If you would like more information on Celcius, read the wikipedia article on Celcius.

Celcius went under some revisions and refinement as the scale centigrade, but in 1948 it was officially renamed Celcius to honur the person who originally thought of it in 1742.

Why switch

Conformity is a good argument, but it isn’t the argument I would use for this. Even though the vast majority of the world uses Celsius,I would say the reason to switch is because it makes more sense and it is easier to see wither or not it is comfortable outside. Refer to the table at the begining. If it’s over 30 °C it’s too hot, if it is under 10 °C then it’s too cold.

Fahrenheit to Celsius Math conversion

This convertion is why I say, just switch outright. Don’t worry about converting. Nearly everything that tells you the temprature can be set to the unit of your choice. If you want to convert this is the official accurate calculation: [°F] = [°C] ×  9⁄5 + 32

If you want something that might be easier to do in your head then this about will get you within 5 °F: Double the °C then add 30. [°F] = 2[°C] + 30

Or don’t bother converting. Refer to the following table.

Fahrenheit to Celsius Table

Fahrenheit	Celsius
-22.0 °F	-30 °C
-4.0 °F	-20 °C
14.0 °F	-10 °C
15.8 °F	-9 °C
17.6 °F	-8 °C
19.4 °F	-7 °C
21.2 °F	-6 °C
23.0 °F	-5 °C
24.8 °F	-4 °C
26.6 °F	-3 °C
28.4 °F	-2 °C
30.2 °F	-1 °C
32.0 °F	0 °C
33.8 °F	1 °C
35.6 °F	2 °C
37.4 °F	3 °C
39.2 °F	4 °C
41.0 °F	5 °C
42.8 °F	6 °C
44.6 °F	7 °C
46.4 °F	8 °C
48.2 °F	9 °C
50.0 °F	10 °C
68.0 °F	20 °C
86.0 °F	30 °C
104.0 °F	40 °C
122.0 °F	50 °C

Thats it, empower yourself. Celsius is easy. Just do it!