empty, sky, void, vacant, vacuum
空 centers on the idea of emptiness or hollowness: a space that is vacant, a void, or the open sky. The meanings 'empty', 'vacant', and 'vacuum' are direct expressions of this emptiness, while 'sky' is the vast empty space above.
sky; the air; the heavens
emptiness; being empty
air; atmosphere
space; room; airspace
airport
aviation; flying
to become less crowded; to thin out; to get empty
(in) the air; midair; sky
aircraft carrier
blue sky
aircraft
sky; the skies; high-altitude sky; upper air
air force
fictitious; imaginary; fanciful; fabricated
empty stomach; hunger
vacuum
starry sky
karate
space; room; gap; emptiness
daydream; fantasy; fancy; vision
blank space (in a document)
air-raid
aerial bombing
empty; vacant; hollow
空 combines 穴 (hole, cavity) and 工, which likely acts as a phonetic component. The combination suggests a hollow space or emptiness.
A hole (穴) with a tool (工) inside: imagine a worker digging a hole, creating an empty space. That emptiness is the sky above or a vacant void.
For クウ, picture a cool empty sky: cool -> クウ, and the vast blue emptiness stretches overhead.