As with most matter, when air is heated, it expands, causing it to become less dense.
(air; is; heated) (it; expands; T:when air is heated) (it; causing; it to become less dense) Context(it causing,List([41, 60))):(it; to become; less dense)
it causing,List([41, 60))
air it it it
is expands causing to become
heated
T:when air is heated
it to become less dense
less dense