Cloud cover
Clouds & visibility cloud amount sky cover
Cloud cover is the fraction of the sky covered by cloud. It describes area, not cloud thickness or the probability of rain.
How much sky is still clear?
Cloud cover divides the visible sky into shares, expressed as per cent or eighths, called oktas. Four oktas means roughly half the sky is cloudy. Zero means cloudless and eight means completely overcast.
Do not add the layers together
A forecast may list low, middle and high cloud separately. Those layers can overlap. Sixty per cent low cloud plus sixty per cent high cloud does not make 120 per cent of the sky. There is only so much sky available.
Overcast does not always mean equally dark
- Thin high cloud: May cover large areas while letting plenty of light through.
- Thick layered cloud: Can produce a much gloomier day.
- Separate cumulus clouds: Leave sunny gaps but briefly darken the ground when they cross the sun.
What the value tells you
High cloud cover does not automatically mean rain. Droplets or ice crystals must grow enough to fall. For a sunny afternoon, cloud type, thickness and development also matter. A model value represents an area, while a window reveals only part of the sky.