Fig. 2 S'n2 chart fm 18ooGMT on 16 February 1996

plays occur when the angular distance of the cloud fiom the sun is less than 40".

The synoptic situation on 16 February was a classic one for the observation of nacreous clouds but they are not seen every time such a situation occurs. During the day a deepening depression to the north of Scotland tracked east into Scan- dinavia leaving Scotland in a very strong and

showery westerly airstream but eastern Scotland was under relatively clear skies (see Fig. 2).

Friday 16 February had all the right ingredients for Mother Nature to exhibit another of her many meteorological wonders and to give the locals of Aberdeen a topic to discuss for some time to come.

Nacreous clouds

Paul Hudson Leeds Weather Centre

Nacreous cloud is rare, stratospheric cloud, also referred to as 'mother-of-pearl cloud' (Meteoro- logical Office 1982,1991), and was first noted and so named by Mohn. When the sun is at a certain critical angle, striking iridescence occurs, causing a beautiful display of colours. The fiont cover shows a good example of such a display.

In 1890 Stormer made reference to nacreous clouds in southern Norway. Later, in 1911, Sir George Simpson (a meteorologist on Scott's last

expedition) drew attention to the fact that this optical effect did not produce halo phenomena and, therefore, the cloud was unlikely to consist of ice crystals. Indeed, with such brilliant colours being produced it is difficult to believe it was anythmg else but iridescence through water drop- lets. Some observations suggest that nacreous clouds consist of 'supercooled' water droplets, so called because air temperatures at altitudes of 18- 3 0 h are usually as low as -90°C. It is not known why the drops can remain liquid at such low temperatures and complicated physical chemistry is involved. Another theory is that the sunlight is dispersed through 'normal' water droplets. A sharp temperature inversion occurs

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within the stratosphere due to photolysis of ozone molecules by ultraviolet radiation. This exother- mic reaction releases heat into the atmosphere which prevents water from freezing and is the only way that water can exist at such altitudes. However, the ‘warm region’ within the strat- osphere is thought to be at an altitude of about 50km, which does not agree with calculations made by Stormer (see below). Calculations made from earlier observations estimate that the diame- ter of the cloud particles did not exceed 0.0025 mm and that the associated optical effects suggest diffraction by spherical particles.

Nacreous clouds have been observed mainly in Scotland and Scandinavia, but have occasionally been reported from France in winter periods with a strong, homogeneous west to north-westerly airstream, with winds consistent with height throughout the troposphere. According to mea-

surements made by Stormer (Meteorological Of- fice 1982, p. 168), the clouds in southern Norway occurred at an altitude of 21-3Okm (70000- 100 000 ft) . The temperatures at this height sug- gest that such clouds are only likely during De- cember, January and, less likely, February. At the same time an extensive and deep depression lies over northern Scandinavia. The clouds are some- what lenticular in form and very delicate in structure, with colouring that lasts for some con- siderable time after sunset. The clouds show little or no movement, giving a strong suggestion of mountain wave activity.

References Meteorological Office (1982) Observer’s h a n d h k .

~ (1991) Mereomlopal ghssay. HMSO, London, HMSO, London, vi + 22Opp.

335pp.

Readers’ Forum

A fragmenting thundery front The general synoptic situation on the morning of 26 July 1995 revealed a thundery low in the south- west approaches with an arc of heavy shower and thunderstorm activity along it. The 0739 GMT satellite image (Fig. 1) shows this well and the front provided 1.27in (32.3mm) of rain in the Poole area. Much of the Midlands thus looked forward to a spell of welcome rain.

The 1236GMT satellite image (Fig. 2) revealed that this front now stretched in a band from North Wales through the West Midlands to the London area, as indeed was proved by the state of the sky here in Halesowen. Early sunshine had given way to a full cloud cover consisting, in the main, of broken cumulus with embedded cumulonimbus. However, no thunder was heard.

A careful watch was kept on the logger system which revealed some quite unusual movements in temperature, humidity and pressure, particularly in the period from 1OOOc;Mr until around 1600GMT (see Fig. 3). The print-out from the logging system here shows these fluctuations well. However, no measurable rain materialised from this system, and no thunder was heard. A few

spots of rain occurred around 1340GMT but this was all. By 16ooGMT the sky had all but cleared.

I would appreciate as much of an explanation as is possible of the unusual fluctuations shown on these graphs, and also the apparent collapse of what had seemed a very active thundery frontal system.

Halesowen, West Midlands

C. G. Roberts

Nick Grahame replies: The thundery front referred to above originated over Iberia and migrated northwards across the Bay of Biscay on 25 July. Daytime heating over Brittany and Normandy injected further thundery activity into the system during the latter part of the day and the feature arrived on the south coast of England soon after dawn on 26 July to give locally torrential downpours as already noted (see also Fig. 4 (a), p. 379). The upper-air radiosonde network showed a marked trough at 250mbar lying from Cornwall to northern France a t 0600 GMT, with a plume of warm air just to the north where 850 mbar wet-bulb potential temperatures exceeded 18°C. At this stage, the positive vorticity advection ahead of the northward-moving upper trough provided the dynamical uplift of the un- stable airmass (Young 1995).

The radar sequence throughout the morning and early afternoon shows how the system subse- quently decayed (Figs. 4 (b), (c) and (d), p.379).

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