Which type of cloud is associated with days of light, continual rain?

The nimbostratus cloud is the continuous rain cloud. The cloud is a low level group and rises up to about 2,000 meters. The cloud consists primarily of water droplets, but may contain ice particles if the surface temperature is cold enough. These clouds are dense enough to block out the sun and may include additional cloud types underneath the nimbostratus layer such as stratus fractus clouds. They range in color from medium to dark gray. The nimbostratus cloud has no definite base and is usually along the front of a weather system.

Nimbostratus clouds typically form by the thickening of altostratus clouds through compaction of layers. Once the altostratus clouds become thick and compacted with water droplets, the weight of the cloud sinks from 2,000-5,400 meters to below 2,000 meters. Nimbostratus clouds may also form from cumulonimbus clouds when wind patterns sheer the higher elevation of the cumulonimbus, above 5,400 meters.