Marginal VFR weather in most airspace is ceilings as low as 1000 feet and/or visibility as low as 5000 metres. Few pilots routinely fly VFR in this kind of weather, so let’s consider go/no-go decision-making when faced with these conditions. (By day, when VMC does not exist, ATC may issue a special VFR clearance, provided the flight is conducted clear of cloud and visibility is not less than 1600 metres for an aeroplane and 800 metres for a helicopter.)