This is not the best way to convert between Fahrenheit and Celsius! But it is the quickest, especially if you are doing the math in your head.
F = 2C + 30
This means that the temperature in Fahrenheit is equal to 2 times the degree in Celsius + 30. So if it is 10 degrees Celsius then 10 doubled is 20 and then 20 plus 30 is 50 Fahrenheit.
You can also convert the other way with the equation below.
C = (F – 30)/2
Warning! This is not exact at all.
The real equations are F = (9/5)C + 32 and C = (5/9)(F-32)
So if the AWOS gives you the temperature in Celsius and you just want to decide if you will need a sweater, then go ahead and use this rule of thumb.
If you need the information for calculating performance, then this rule of thumb is inappropriate to use and there is a much better way.