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- three possibilities:
a) Altocumulus in two or more layers, not increasing
b) Altocumulus opacus in a single layer, not increasing
c) Altocumulus, together with Altostratus or Nimbostratus

graphic
in this picture Altocumulus at different levels

graphic
in this picture sheet of Altocumulus Opacus (right half of the photo)

graphic
in this picture Altostratus with near the horizon elongated elements of Altocumulus

graphic
in this picture lines of Altocumulus in the distance and thin Altostratus overhead

from the top downward:
photo by R.K. Pilsbury
photo by T. Bergeron
photo by A.J. Aalders
photo by R.K. Pilsbury (Crown copyright)