air alert vs warning signal

air alert and warning signal both are nouns.

Word	Noun	Adjective	Verb	Adverb
air alert	Yes	No	No	No
warning signal	Yes	No	No	No

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

air alert: the warning signal that begins a period of preparation for an enemy air attack
warning signal: an automatic signal (usually a sound) warning of danger

Other hypernyms of air alert include alarm, alarum, alert.

air alert (noun)	warning signal (noun)
the warning signal that begins a period of preparation for an enemy air attack	an automatic signal (usually a sound) warning of danger
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