Celsius and Fahrenheit

I've changed my weather preferences on the web sites and apps that I check the weather on (my.yahoo.com, weather.com and my iphone) to show the temperature in Celsius. My original idea was to learn what Celsius really means, and it basically worked.

The exact conversion formula is to multiply the Celsius temperature by 9, divide the answer by 5, and add 32 to get the exact Fahrenheit measure.
An easier way to get an approximation is to double the Celsius reading and add 30, however that's still too complicated.
The easiest way is to follow my simple rule of thumb:
30's = Hot
20's = Warm
Teens = Cool
Below 10 = Cold

There are plenty of online tools, however I think it's easiest to Google "Convert 30 degrees Celsius to Fahrenheit" to get "30 degrees Celsius = 86 degrees Fahrenheit, which is where hot starts in my book.

UPDATE: A friend turned me on to this easy rhyme which I think would have been based on my simple rule, had it not been invented decades ago to teach British schoolchildren. It's even on a T-shirt from Go Metric:
100 is boiling
30 is HOT
20 is NICE
10 is COOL
0 is ICE