move your mouse over the the image
Note the 'hole' in the altocumulus clouds moving in from the left (west).
Obviously there is 'something' falling out which also generates a
halo EE13 ("Horizontal Kreis") or parhelic circle (sorry some links in german - but the best i could find).
Accordingly this 'something' were horizontal ice-plates or horizontal columns although
it looks more like falling snow, i.e. larger ice crystals. Interestingly the AC clouds show no halo.
Unfortunately it was not in zenith i.e. not observed by our vertical pointing instruments,
except for the cloud base of the AC deck which was around 8km.
But you may want read Westbrook and Davies (2010) or Heymsfield et al. (2010).
This phenomen is called a fallstreak hole or punch-hole-cloud.
Images made every 20sec with an AXIS M3027-PVE Fixed Dome Network Camera,
North is on top, east on the left. For details see this page.