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A METAR reports a visibility of 8 km. Added to the report is a "BECMG 4000". What can you expect to happen to the visibility?
  • A
    Improve from 4 000 m to 8 000 m in the next 2 hours.
  • B
    Vary between 4 and 8 km.
  • C
    Decrease to 4 km within the next 2 hours.
  • D
    Decrease to 4 km within 1 hour.
METAR and SPECI are observations of actual weather conditions. When a change is expected to occur within the next 2 hours, the trend forecast shall begin with one of the change indicators "BECMG" or "TEMPO":
  • TEMPO. Temporary change between the specified period. After the end of the period, the weather will revert back to the state of the last definitive change
  • BECMG. Gradual change from a previous state to a new one between the specified periods. After the end of the period, the weather will be the described one, nullifying previous changes.
    "BECMG 4000" => visibility will become 4 000 m within the next 2 hours.

ICAO Annex 3
Appendix 5
1.3.4 Recommendation.- The change indicator "BECMG" and the associated time group should be used to describe changes where the meteorological conditions are expected to reach or pass through specified threshold values at a regular or irregular rate and at an unspecified time during the time period. The time period should normally not exceed 2 hours but in any case should not exceed 4 hours.

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