air alert vs period of time

air alert and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
air alert	Yes	No	No	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of air alert; that is, period of time is a word with a broader meaning than air alert:

air alert: the time period during which military and civilian agencies are prepared for an enemy air attack
period of time: an amount of time

Other hypernyms of air alert include period, time period.

air alert (noun)	period of time (noun)
the warning signal that begins a period of preparation for an enemy air attack	an amount of time
the condition in which combat aircraft are airborne and ready for an operation
the time period during which military and civilian agencies are prepared for an enemy air attack