The word airmass is a singular noun. The plural is airmasses.
Air does not remain in a given place. A country does not have an 'airmass'.
The warmer airmass that was coming would mark a positive change in the weather.
An airmass that is very warm and dry would be classified as a continental tropical (cT) airmass. These airmasses form over hot and arid regions, such as deserts, and bring warm and dry conditions to areas they move into.
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.