CAVOK

by Dave Ashford

Have you ever looked at a METAR that says CAVOK and thought to yourself "Great! No clouds below 5000 feet, no CBs and visibility in excess of 10 kilometres!"? If so, read on - you may wish to change your mind.

During my PPL training my instructor taught me the definition of CAVOK and then immediately said that all I could conclude from a report of CAVOK was that the cloud base was above 1500 feet, visibility was no better than 5000 metres, there was no significant rain and there were no CBs in the area. He said I could not assume that there was no cloud below 5000 feet nor that the visibility was in excess of 10 kilometres because a CAVOK report was not changed until there was significant cloud below 1500 feet and/or visibility dropped below 5000 metres. Trevor Thom said much the same, so I accepted it, even though it didn't sound right. Recently I had the opportunity to get a clarification of CAVOK from the Met Officer for Civil Aviation Services at Bracknell, and here is his reply:

Interpretation of CAVOK in Aeronautical Reports and Forecasts

Met Office document cavok.doc.3/96 by Anthony D'Aubyn

Refs: UK AIP (MET) table K and AIC 79/1988

A weather report (METAR) of 'CAVOK' must have the following criteria satisfied simultaneously:
- visibility not less than 10km, no reportable weather significant to aviation, no cloud below 5000FT and no CB at any level.
In a TAF, a forecast deterioration from CAVOK has to be inserted if the following are expected to occur:
- visibility less than 10km, onset of significant weather, CB developing, or if more than half cover of cloud (BKN or OVC) develops at, or lowers to, a level below 5000FT.
In a TREND (2 hr landing forecast appended to selected METARs), changes in visibility and cloud are not regarded as 'significant' until the vis falls to 5km or less, and/or the base of the ceiling (lowest BKN or OVC layer) falls to 1500FT or below.

Thus after a report of CAVOK in a METAR, the TREND can legitimately state 'NOSIG', but only if it is expected in the next two hours the vis will be no lower than 6km and the ceiling will be no lower than 1600ft.

Summarising:

A report of CAVOK must always satisfy the criteria.

A TAF should indicate vis changes through 10km and ceiling through 5000FT, CB, and the onset of intenser forms of weather (e.g. moderate or heavy precipitation, thunderstorms). Less intense forms of weather (e.g. slight precipitation, mist), and cloud amounts of half cover or less (FEW/SCT) are not regarded as significant enough to require a change in the forecast.

A TREND, however, will not indicate a deterioration from CAVOK unless the vis is expected to fall to 5KM or less, the ceiling to 1500FT or below, and/or intenser forms of weather are likely to occur, within the two hour period immediately following the report.

Thus, for example, a TAF might say:

          23010KT CAVOK BECMG 0911 8000 -RA BKN030

but the METAR and TREND at 0850Z would still be consistent in stating:

          23010KT CAVOK NOSIG

and both TREND and TAF would be 'correct' (perhaps better described as being 'within the tolerance allowed'!) if the METAR at 1050Z happened to be:

          25015KT 6000 -RADZ FEW008 SCT012 OVC016