M

• in RVR indicates visibility less than lowest reportable sensor value (e.g. M600)

  METAR

• in temperature field means “minus” or below zero

  METAR

• Mach number

  General

• maritime (air mass)

  Weather

• metres (preceded by figures)

  ICAO

• missing (weather reports only)

  Weather

• Mountain Standard Time (time groups only)

  Weather

M has 7 meanings; the right one depends on context. A weather report, a NOTAM or an ATC clearance. Got a NOTAM full of contractions? Paste it into the decoder.

Source: FAA Order JO 7340.2 Contractions. Not for operational use.