Why are sound waves in air characterized as longitudinal?
A longitudinal wave is one which moves in the same direction as teh medium, like sound in air.
The medium defines if the wave is longitudinal or transverse.
A plucked violin string is an example of a transverse wave as the medium -- the string -- moves up and down.
This up/down motion of the string compresses and uncompresses the air which propagtes the sound in that direction: so is a longitudinal wave.