I’m a meteorologist and author (Weather Whys: Facts, Myths, and Oddities), and I know I’ve said it, but, technically, temperatures cannot be warmer or colder.
A temperature is a number, which means that it can be higher or lower, not warmer or colder. One hundred is not warmer than 20, and 20 is not colder than 100.
As far as the air is concerned, it is indeed warmer at 100 degrees than it is at 20 and colder at 20 degrees than it is at 100; however, the numbers themselves are either higher or lower. Therefore, technically, we should say that the temperatures are higher or lower, not warmer or colder.