ACCAST

(Altocumulus castellanus or castellatus; the proper abbreviation for the Daily Register is 'Ac cas'): These are medium level clouds (between circa 8000 and 18000 ft; 2.5 to 5.5 km) which exhibit, at least in their upper parts, marked cumuliform / turreted appearance - the convective towers are often taller than the width of the individual base; the bases are connected and often appear to be arranged in lines or streets. These clouds are a good indicator of medium level instability and high moisture content - and often the precursor for widespread thundery activity within the following 24 to 48 hours. Observers used to SYNOP coding may also refer to these (along with Altocumulus floccus/Ac flo), as CM8 clouds.