In meteorology, Altocumulus castellanus or Altocumulus castellatus
(ACCAS) is a cloud type named for its tower-like projections that billow upwards from the base of the cloud. The base of the cloud can form as low as 2,000 metres (6,500 feet), or as high as 6,000 metres (20,000 feet). They are very similar to cumulus congestus clouds, but at a higher level and with the cloud heaps joined at the base.
Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude
lapse rate
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'' (in its "becoming less" sense, not its "interruption" sense). In dry air, ...
.
They may be a harbinger of heavy showers and thunderstorms and, if surface-based
convection
Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
can connect to the mid-
tropospheric
The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary s ...
unstable layer, continued development of Castellanus clouds can produce
cumulonimbus
Cumulonimbus () is a dense, towering, vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water va ...
clouds.
Altocumulus castellanus clouds are typically accompanied by moderate
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
as well as potential
icing conditions
In aeronautics, icing is the atmospheric icing, formation of water ice on an aircraft.
Icing has resulted in numerous aviation accidents and incidents, fatal accidents in aviation history.
Ice accretion and accumulation can affect the external s ...
. For these reasons, flight through these clouds is often best avoided by aircraft.
References
External links
Altocumulus Castellanus (Source: Australiasevereweather.com)
{{Cloud types
Cumulus
Gliding technology