Cold versus Warm years

[Michael Hurd]

For the weather data for the United States, what exactly do the warm and cold year choices represent? I seem to remember reading or hearing that these represent the 10% warmest and 10% coldest years taken from 30 years of weather data. Please confirm. Also if my assumption is correct how is this arrived at? By evaluating the average yearly temperatures or the means of the yearly temperatures? Or, some other way?