Here's why:
* Proper nouns are names of specific people, places, or things. They are always capitalized. Examples: John, New York City, the Eiffel Tower.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples: man, city, tower.
"Air temperatures" is a general term referring to the temperature of the air. It's not a specific name. Therefore, it's a common noun.