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|Content control:||Royal Meteorological Society (RMetS)|
Fog comprised of supercooled water droplets which normally deposit rime ice on contact with an object with a surface temperature of less than 0°C32 °F <br />273.15 K <br />491.67 °R <br />. Fog is generally defined in terms of visibility being less than 1000m horizontally and freezing fog typically forms in the temperature range of -0°C32 °F <br />273.15 K <br />491.67 °R <br /> to -20°C-4 °F <br />253.15 K <br />455.67 °R <br />. 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°C14 °F <br />263.15 K <br />473.67 °R <br /> to -20°C-4 °F <br />253.15 K <br />455.67 °R <br /> 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.