buoyancy vs blitheness

buoyancy and blitheness both are nouns.

Word	Noun	Adjective	Verb	Adverb
buoyancy	Yes	No	No	No
blitheness	Yes	No	No	No

As nouns, blitheness is a hypernym of buoyancy; that is, blitheness is a word with a broader meaning than buoyancy:

Other hypernyms of buoyancy include cheerfulness.

buoyancy (noun)	blitheness (noun)
the property of something weightless and insubstantial	a feeling of spontaneous good spirits
irrepressible liveliness and good spirit
the tendency to float in water or other liquid
cheerfulness that bubbles to the surface