Widespread cloud cover with the altostratus cloud blotting out the sun completely hence opacus. However, undulations were observed within this cloud cover. The other photo perhaps illustrates these in more detail. This is rather different from altocumulus undulatus because it involves the lifting of a large airmass leading to widespread condensation. This condensation combined with wind shear at the cloud base (clearly shown on the weather sounding) create turbulence within the cloud layer hence the cloud appearing like a roll. For this to happen within altostratus, the airmass needs to be very humid as was in this case.