half-hourly vs periodic

half-hourly and periodic both are adjectives.

Word	Noun	Adjective	Verb	Adverb
half-hourly	No	Yes	No	Yes
periodic	No	Yes	No	No

half-hourly (adjective)	periodic (adjective)
occurring ever half hour	happening or recurring at regular intervals
	recurring or reappearing from time to time

half-hourly (adverb)	periodic (adverb)
every thirty minutes, every half hour