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