Fog comprised of supercooled water droplets which normally deposit rime ice on contact with an object with a surface temperature of less than 0°C. Fog is generally defined in terms of visibility being less than 1000m horizontally and freezing fog typically forms in the temperature range of -0°C to -20°C. Freezing fog is reported as FZFG on a Meteorological Terminal Air Report (METAR) report.
Surface contact with subzero surface temperatures notwithstanding, water droplets will continue to exist in supercooled liquid form until the temperature drops below -10°C to -20°C at which time the droplets are likely to freeze and become ice crystals (see Ice Fog).
Freezing Fog will leave only a thin film of frost or rime ice on an aircraft, which nevertheless needs to be removed before flight with a de-icing treatment. Aircraft should also receive an anti-ice treatment before commencing a take-off if freezing fog conditions exist for the taxi and take-off phases of flight in order to prevent further build-up of ice.