Cloud that will obscure the sun, they are layered without features and often dark, grey. They can produce persistent rain.
If its a dull, wet day then its a good bet that nimbostratus are above. Thick enought to blot out the sun and able to produce persistent rain.
What's in a name?
Nimbus - rainy, rainstorm. Stratus - to spread out, extend, flatten, layer.
How was it made?
Nimbostratus clouds form when altostratus cloud deepens and thickens. This often happens along warm fronts.
What weather will you have if these are overhead?
It will be wet, persistent rain or snow, defenitely in for a soaking..
How high is it?
Anywhere from 700m (2000ft) up to 3500m (10,000 ft).
The outcome for you.
It's going to be a a wet day and is likely to set in for a considerable time.