Nimbostratus clouds ( these clouds we see a lot over Cornwall!!!)
Nimbostratus clouds are dark, grey, featureless layers of clouds, thick enough to block out the sun. Producing persistent rain, these clouds are often associated with frontal systems provided by mid-latitude cyclones. These are probably the least picturesque of all the main cloud types.
Nimbostratus clouds are featureless, very dense and have little characterisation, so are not classified into 'species' like other cloud types. They are similar to altostratus in this way.
These mid-level clouds are often accompanied by continuous moderate rain or snow and appear to cover most of the sky. Nimbostratus will often bring Rain which may last for several hours until the associated front passes over.
If there is hail, thunder or lightning present it is a cumulonimbus cloud rather than nimbostratus