Castellanus

A castellanus (or castellatus) (from latin castellanus, castle) is a cloud that displays at least in its upper part cumuliform protuberances having the shape of turrets that give a crenellated aspect. Some of these turrets are higher than they are wide; they have a common base and seem to be arranged in a line. The castellanus characteristic is particularly obvious when the clouds are observed from the side (i.e., from a vantage point on a line perpendicular to the line of orientation).

It is a cloud species attached to the cloud genera cirrus, cirrocumulus, and . Species of the clouds include cirrus castellanus, cirrocumulus castellanus, and . Sometimes cumulus castellanus are referred to, but the type is not recognised by the France's national meteorological service Météo-France, or by the American Meteorological Society and World Meteorological Organisation. Those clouds some would classify as cumulus castellanus typically do not have a common base and are not arranged in a line, thus differing to some extent from the more universally-recognised castellanus types. Some scientists also think that the castellanus should be a full cloud genus, and not just a cloud species. The Federal Aviation Administration implicitly considers a castellanus as a full cloud genus.

All castellanus clouds show that there exists an unstable (or conditionally unstable) layer at their altitude but not necessarily under the cloud. Some scientists (Scorer, Corfidi) define a castellanus as a cloud generated by the release of latent heat during the ascension of a saturated thermal column in an unstable (or conditionally unstable) layer at altitude. The cloud will have the appearance of a ''turkey tower''. This unstable (or conditionally unstable) layer can be generated in different ways: (1) a large scale lifting (synoptic scale) that under some circumstances makes the air unstable since the temperature at the base of the layer decreases more slowly than the temperature at the top of cloud due to adiabatic decompression, (2) a cooling of the cloud top that generates the same differential, and (3) an unstable (or conditionally unstable) airmass advection over a stable airmass, etc.

...
Wikipedia