Interpret cloud cover percentages.
Meteorologists report cloud in oktas — eighths of the sky dome — and aviation codes map directly onto that scale: few, scattered, broken and overcast. A ceiling only exists once cover reaches five oktas, which is why broken and overcast layers matter for flight planning while scattered cloud does not. Cloud reduces incoming solar radiation by roughly three quarters of its fractional cover, with multiple layers attenuating a little more.
Oktas to cover and attenuation
Cover per cent = oktas / 8 x 100; solar reduction = cover per cent x 0.75 x layer factor (1 + 0.08 per extra layer), capped at 90%
It is the fraction of the visible sky dome obscured by cloud, conventionally reported in eighths. Five oktas is 62.5% and is the point at which a ceiling is recorded.
The height of the lowest broken or overcast layer — five oktas or more. Scattered cloud does not constitute a ceiling however low it sits.
Not quite. Thin high cloud transmits a lot of light, and even overcast skies leave meaningful diffuse radiation, so the reduction is less than the cover fraction.