Condign volumes 1 to 3

# UNCLASSIFIED SECRET UK RESTRICTED

- At the top of cumulus clouds to form a 'cap-cloud' (pileous cloud).

- With a 'metallic grey' colour.

4. If cumulus cloud below a cap-cloud evaporates or disperses the lenticular cap can descend (sink). They can remain poised or slowly wavering above hills. Lenticular effects can form with several basic cloud types, e.g. Altocumulus Lenticularis, Cumulus Pileus, Strato-Cumulus Lenticularis etc.

5. Some photographs ( for example, at Figure 2) show that lenticularis clouds (or 'wave clouds') can appear as billows (i.e. ovals/lens-shaped discs with a hole in the centre) and also with laminated appearance. Pileous laminations occur when humid layers are sandwiched between dry air layers. At Figure 5 little imagination is required to describe 'a dome on top'. When the top of a wave-cloud is 'bow-shaped' and smooth, this usually indicates that the air above is stable and dry. The size of lenticularis clouds may extend for kilometres. However, small ones seen edge-on may give the impression of much smaller discs. These formations often appear to the leeward side of high ground. The typical reported altitude might be 3000- 6500 metres.

## MAMMA

6. These protuberances occur when small-scale convection changes cause portions of cloud to sink in clear air below a cloud formation. The sinking elements mix with the clear air and usually evaporate. However, if they contain rain or snow crystals, the shapes shown at Figure 6 can occur. These particularly occur beneath the

thick clouds associated with thunderstorms. Mamma can also occur at the lower ends of cirrus cloud formations.

## NOCTILUCENT CLOUDS

7. Noctilucent clouds have the appearance of decayed cirrus but are typically at 80Km altitude in the stratosphere. They can shine for an hour or so after sunset but they are not seen if the sun is above the horizon. If the sun is more than $ \sim 1 6^{\circ} $ below the horizon noctilucent effects cannot be seen. Normally noctilucent clouds appear between May and August at latitudes $ 5 0^{\circ}-7 0^{\circ} $ N. (This includes UKADR latitudes). It is reported that these may be caused by dust particles which catch the lower stratosphere during volcanic eruptions, or due to dust produced in the solar system by collisions among minor planets. Noctilucent effects can also be caused by terrestrial-based light sources shining on the underside of disc-shaped or laminar clouds. Noctilucent clouds are the highest clouds in the atmosphere.

## MOTHER OF PEARL CLOUDS

8. 'Mother of Pearl' formations (Figure 9) are rare but can be seen in Scotland occasionally between November and March. They have iridescent bands of colour, lit by sunlight in a twilight sky (limiting the time of occurrence). They occur only at altitudes of the order 2230Km and probably comprise super-cooled droplets with ice crystals.

## DUST DEVILS, TORNADOES & WATER SPOUTS

9. These occur in unstable layers of dry air over hot ground. They are visible because of the dust (which may rotate in