Genius of the lamps
On a September evening last year, my husband took this photograph (main picture, above) outside the front of our house in Cambridge, UK, looking east. The clouds seemed to have come out of Aladdin鈥檚 lamp. Can someone explain how such a remarkable pattern can be formed, or it the genie鈥檚 tale true?
鈥� Something remarkable has appeared from this genie鈥檚 lamp: The Last Word has discovered that we have our own All-Seeing-Eye in place without even having had to wish for it.
Within a few days of the original photograph of the 鈥淎laddin鈥檚 clouds鈥� being published, we received three more photographs taken of the same five clouds on the same day from different locations in eastern England.
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One of them (bottom right of the four smaller photographs) was taken by Rowan Moore at Dunchurch, near Rugby, 100 kilometres to the west of Cambridge; one (bottom left of the four) was taken by Martin Williams at Holme, near Peterborough, 40 kilometres north-west of Cambridge; and the third photograph (top right of the four), by Clive Semmens in Ely, 25 kilometres north of Cambridge.
The final picture (top left of the four) is of a larger group of similarly shaped clouds photographed from Nottingham, in central England, by Sean May on a different date. The explanation for these peculiar clouds has been provided by another reader鈥� 鈥� Ed
鈥� These strikingly shaped clouds are not rising and trailing particles below them, as their appearance in the photograph suggests. They are actually altocumulus clouds from which precipitation is falling.
Such trails of water droplets or ice particles are called virgae (or sometimes fallstreaks) and, by definition, do not reach the ground. Precipitation that does reach the Earth鈥檚 surface is known technically as praecipitatio.
Virgae are produced from what are known as heads. These can be distinct, rounded clumps of cloud like the ones that are shown here, ragged tufts, or extremely small patches of cloud that are difficult to distinguish from the tops of the virgae themselves. They occur at all cloud levels, and ordinary cirrus clouds 鈥� the thin wispy clouds that are popularly called mares鈥� tails 鈥� essentially consist solely of virgae.
Virgae often show distinct bends like the ones clearly visible in the photographs. These bends often occur where the falling ice particles melt into water droplets.
The ice crystals fall almost vertically. But as the water droplets start to evaporate, they become smaller and so fall more slowly, leaving them trailing behind the cloud above.
In other cases, the bend may indicate a region of wind shear, where the strength or direction of the wind changes. On very rare occasions, where the wind is stronger at a lower level, virgae have even been observed in front of the head that generated them.
Storm Dunlop
Chichester, West Sussex, UK