(meteorology) According to the World Meteorological Organization, a mid-level, principal cloud type, generally formless and dark grey in colour, which forms from altostratus occurring in layers at the middle altitude of the troposphere (usually above 2400 metres). Nimbostratus usually brings precipitation as the mid-level clouds thicken and subside into the low level of the troposphere. Frontal or cyclonic lift can also carry the top of a deep nimbostratus layer into the high levels of the troposphere. Also classified or characterized as multi-level; abbreviated Ns.