high, low
Hyphenate as compound modifier preceding a noun; do not hyphenate following a noun unless in Webster's:
the high-energy photon was observed
the knee-high muck surrounded us
he stood knee-high in shavings
low-density sprawl
but the cartons were stacked three high
that was a low blow
Lowercase for a weather system:
the Aleutian low
a low weather system
a Bermuda high