M2 Clouds: Thick altostratus (As) or Nimbostratus (Ns)
or
Nimbostratus.
Notes: M2 altostratus is denser and of a darker gray or bluish gray than M1 altostratus with the greater part sufficiently dense to completely mask the sun or moon. With further thickening of the altostratus and a lowering of its base, the cloud may begin to produce precipitation at which point it is called nimbostratus.
Some cloud charts will depict nimbostratus as a low-level cloud. This is because often, during continuously falling precipitation, the base of nimbostratus clouds decreases into the low etage. But officially and historically, nimbostratus is classified as a mid-level cloud.
Also, continuously falling precipitation, L7 clouds form beneath the base on nimbostratus and may even merge with the M2 cloud. To tell the difference between the two, L7 clouds are generally moving fast and changing shape rapidly.
