In meteorology, cloud cover is measured in oktas. The unit of cloud amount – okta – is an eighth of the sky dome covered by cloud. Sky needs to be mentally divide it into eight boxes, then all the visible clouds are squashed into these boxes.
The number of filled boxes is how many oktas of cloud there are.
Cloud amounts are generally round up to the next okta. For example ‘2 and a bit’ oktas is rounded to 3 oktas.
The exception is when more than 7 but less than 8 oktas is observed – in this instance cloud amount is rounded down to 7 oktas.
- 0 oktas represents the complete absence of cloud,
- 1 okta represents a cloud amount of 1 eighth or less, but not zero,
- 7 oktas represents a cloud amount of 7 eighths or more, but not full cloud cover,
- 8 oktas represents full cloud cover with no breaks,
- 9 oktas represents sky obscured by fog or other meteorological phenomena.
IMAGE: Ahmad etal Experimental Agrometeorology – Scientific Figure on ResearchGate. Available from: Link.