I came across these extraordinary clouds (right and below) while I was flying from London to Grenoble on the morning of 23 December 2005. The pictures seem to indicate chimneys belching steam into the air at cloud level yet the cloud base was seemingly far too high. The two images are from different angles. What do they represent?
The clouds shown in these images are indeed produced by chimneys belching water vapour, although these chimneys are not at cloud level. On this occasion high pressure has created a temperature inversion. This is where, in a change from the norm, air temperature is higher at higher altitude. Descending air is warmed and traps any colder air at ground level.
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There will be very little wind in the cold air mass and the steam belching from chimneys will rise straight through the cold winter air. Because the steam from the chimney is warmer than the upper air mass it can break through the temperature inversion into the warm air above it.
It is no coincidence that the temperature inversion is at the same level as the cloud. Where the warm and cold air masses meet, the cold air causes water vapour to condense and form the blanket of stratus cloud through which the steam emerges.
A quick check of archives for the day in question at reveals that the necessary high pressure was present. Further pictures of this phenomenon can be seen at the gallery under 鈥減yrocumulus and fumulus鈥�.
Christopher White, Newbury, Berkshire, UK
In my 20 years of flying a Royal Air Force Vulcan from a base in Lincolnshire near England鈥檚 east coast, I witnessed the phenomenon shown in the photograph a number of times.
During the winter, with a light easterly or north-easterly wind, a sheet of stratus cloud would drift in from the North Sea over the land. Its base was between about 200 and 270 metres while the top of the cloud sheet was always well defined, falling between 660 metres to 830 metres. At this cloud top there was a strong temperature inversion and the temperature increase in the clear air immediately above was sometimes as much as 8 掳C.
On such days, hot air from the cooling towers of the three electricity-generating stations on the river Trent would burst through the inversion and stream downwind in the manner shown.
John LeBrun, H茅rault, France