* Dense, thick clouds: These are usually cumulonimbus clouds, which are associated with thunderstorms. They are so thick and dark that they completely block the sun.
* Any cloud type with high coverage: Even a thin layer of clouds, like cirrostratus, can block the sun if they cover the entire sky.
* Clouds with a high opacity: Clouds can vary in opacity, meaning how much light they allow to pass through. A dense stratus cloud will block more sunlight than a thin altostratus cloud, even if they cover the same amount of sky.
You could also use descriptive terms like:
* Overcast clouds
* Dark clouds
* Sun-blocking clouds
Ultimately, the best way to describe the clouds blocking the sun depends on the context and what you want to emphasize.
