Ns compared with As op

(Section 2.3.6.6.1)

 Nimbostratus is distinguished from Altostratus opacus (thick) by:

• Generally having a darker grey colour than Altostratus.
• Being thick enough throughout to hide the Sun or Moon; only the thicker parts of Altostratus opacus hide the Sun or Moon. On moonless nights, there may be doubt regarding the identification of Nimbostratus or Altostratus. The cloud is identified as Nimbostratus when precipitation (rain, snow  or ice pellets) reaches the ground.
• When discernible, usually having a lower base than Altostratus.
• Sometimes having the appearance of being illuminated from the inside (this feature is never found in Altostratus).