Ns compared with Cb
(Section 2.3.6.6.5)When the observer is beneath a cloud having the appearance of Nimbostratus, but accompanied by lightning, thunder or hail, the cloud is classified as Cumulonimbus.
When the observer is beneath a cloud having the appearance of Nimbostratus, but accompanied by lightning, thunder or hail, the cloud is classified as Cumulonimbus.