Castellanus

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 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) air mass advection over a stable air mass, etc. Castellanus can arise from clouds formed by either surface-based convection (e.g., cumulus) or elevated convection (e.g., altocumulus). Conversely, if the air surrounding the cloud is nearly saturated and a thick, unstable (or conditionally unstable) layer exists above the initial cloud, convection will be able to continue. Then a tower, that will become wider and wider, will build up that will in some cases become a cumulonimbus. The evolution of a castellanus thus depends on the amount of humidity and the characteristics of the unstable (or conditionally unstable) layer. Scattered clouds in the sky are generally found in relatively dry air with a more humid pocket. If they show such a tall and narrow growth, the dry air surrounding them will not perpetuate the convection which will result in the cloud dissipation. However, a more extended cloud layer shows a higher relative humidity in the layer; a scenario more likely to be associated with over-development of the towers. == Nomenclature ==

The species castellanus is defined in the International Cloud Atlas, which had its last official release in 1975. It is based on ground visual observation of cloud shape. In practice, the shape of a castellanus is related to the physical process producing it: local instability (or conditional instability) at the top of the cloud that breaks the temperature inversion and is the cause of the formation of towers, this being confirmed by vertical soundings. However, although the species castellanus applies to different genera of convective clouds at different levels, the most current cloud type is the altocumulus castellanus, the base of which is in the medium level. This is reinforced by the fact that pilot's manuals interchangeably talk about either castellanus or altocumulus castellanus, restricting the meaning of the word catellanus. propose a cloud classification based only on physical criteria obtained from vertical soundings and observations from meteorological satellites to reclassify the castellanus as a cloud genus at the same level as a cumulus or a cirrus. Corfidi has long criticized the resistance from official bodies to this change. However, the 2016 version of the International Cloud Atlas will not change the status of the castellanus. The same scientists have a similar opinion about lenticular clouds that can exist at different levels that are not recognized as a full cloud genus but only as a species. Criticism specifics . View to the south. Scientists note a common confusion between convective clouds generated solely (or at least primarily) by diurnal heating of the ground and the castellanus, which is primarily a product of instability (or conditional instability) at the top of a cloud such as a cumulus, stratocumulus, or an altocumulus, the latter two genera being easily confused. Moreover, the International Cloud Atlas (ICA) is not entirely precise regarding how to determine the genus of a convective cloud. Per the ICA (1975, p. 15–16), the base of a cumulus cloud is usually between the surface and . This height limit originates from observations in temperate climates where the surface temperature and humidity are about average. It does not take into account how the cloud is formed. In the American West, the base of cumulus clouds can reach , since the combination of a low dew point () and high temperature (up to ) will yield a very high convective condensation level. While the ICA allows that a cumulus cloud can have a base higher than (by saying that cumulus clouds usually have their bases at or below this height), guidance given on page 16 of the ICA could lead to misclassification of a high cumulus cloud. This guidance says that a cloud's genus can be determined "by making a choice from among the genera normally encountered in the etage (height range) corresponding to its height." This could lead the observer to rule out cumulus as a possible genus for a cloud that has all the ICA-described characteristics of a cumulus except that its base is above the "Low" etage (surface to ). The observer, conscientiously trying to comply with ICA classification guidance, might then misidentify this cloud as an altocumulus (perhaps altocumulus floccus), although the instability creating it is at a different level compared to an altocumulus. the thunderstorm is located to the east of the picture. The picture's caption in Corfidi's paper refers to the cloud as a castellanus == Soaring ==

.Thes clouds can sometimes be used by glider pilots Castellanus (clouds made of very narrow columns) are notorious as being unusable by glider pilots. In order for a cloud to have a usable thermal, the updraft column needs to exist under the cloud, in which case the cloud will have a flat base. An altocumulus castellanus, however, can be identified by the lack of a well-defined base. As said above, a hybrid cloud between cumulus and castellanus can be used by a glider pilot. Castellanus that are unfavorable to soaring can be easily identified and are nicknamed ''rocket clouds.'' The visual difference between a cumulus and a castellanus is that the former has a flat base while the latter generally has no clearly defined base. However, a visual observation of the cloud may not be sufficient, since in some cases the hybrid castellanus (pseudo cumulus) may have a flat base. The only sure way to distinguish between these two types of clouds is through an atmospheric sounding preceding the flight. If the base of these false cumulus is higher than the convective condensation level, the pilot is probably close to unwanted cumulonimbus. Moreover, when a cumulus breaks down, there may still exist an updraft column under the cloud base that did not originate from the ground. In particular, at the end of the day, the air-mass can be stable up to 2000 feet and unstable above this stable layer. The cumulus turn into stratocumulus or altocumulus depending on their height. Before the 1956 version of the International Cloud Atlas, these clouds were called stratocumulus or altocumulus "vesperalis". Directly under these clouds, updrafts are still present. The meteorologist Corfidi explains that these seemingly innocuous clouds that break down rapidly can be a harbinger of nocturnal thunderstorms. To identify non-ground-based convection, it is possible to compare the height of the base of the "cumulus" c with the convective condensation level h. This height h is given by the following formula: h = a \times (T - D) where a = 0.125 km / °C; T is the surface temperature and D is the dew point. The variables c, T and D can be easily accessed from a neighbouring METAR. The computation of this height h is based on first principles and therefore is rather accurate. Consequently, if c \gg h, the observed cumulus are almost certainly castellanus under Corfidi's meaning. Then, it will be wise to be wary of the presence of a flanking line or of downbursts in the vicinity. Figure 2 shows such a pattern. == Notes and references ==

Notes References == See also ==