"Light / Medium / Heavy" air depends on the skill of the sailor and the conditions they are used to. What most of us in North America call "heavy air" is medium to an Australian.

For a Hobie 16 in North America, this is the generally accepted ranges:
Light air is 0 to 7 knts (no trapezing)
Medium is 7 to 15 (transitional trapezing to solid double trap)
Heavy is 15+ (depower mode)

It also varies by boat. A Hobie 17 is just coming alive in stuff that a 16 would consider "heavy".