I wonder if I upset the feline population yesterday with supercilious talk about their weather-cleverness? Perhaps, as used to be believed was possible, some oriental puss, chasing its tail or playing with a dangling rope, really did succeed in stirring up the wind? In any event, even as a Weather Eye on feline meteorology was being prepared, a CAT of a different kind was causing havoc over the Pacific.
An aircraft, en route at 33,000 feet from Tokyo to Honolulu, hit a patch of very severe "clear air turbulence", known in the trade as CAT. One woman died, and 10 passengers were sufficiently badly injured by it to be kept in hospital.
Clear air turbulence occurs at high altitudes and results from very sharp changes in wind strength or direction over short distances, distances comparable to the size of the aeroplane itself. It manifests itself as a violent buffeting, as spectacular as might be expected in association with the severest thunderstorm. But CAT is all the more alarming in that it is often unexpected. Unlike the turbulence of thunderclouds, it is - as indeed its name implies - unaccompanied by any visual warning of its onset. Luckily, some 95 per cent of CAT reports are of the "light" or "moderate" variety, and only in about 1 per cent of cases is the turbulence described as "violent".
CAT, unfortunately, is very difficult to predict with confidence and is as yet impossible to detect in flight until an aircraft encounters it. It is also ephemeral in nature: it often happens that one aircraft may hit a turbulent zone quite unexpectedly, while one following on exactly the same track may experience no turbulence. Forecasters reckon there is always a background probability of somewhere around 2 per cent that CAT will be encountered, and their efforts at prediction concentrate on identifying those areas where the probability may rise significantly above this figure, and where any incident may be severe.
It is particularly common at the edges of the "jet stream", a narrow ribbon of very strong winds that undulates around the globe in the middle latitudes between 25 and 40 thousand feet above the ground. The winds at the core of the jet stream typically blow at 100 to 150 m.p.h., and winds as strong as 300 m.p.h. have been experienced.
This slender tube of high-speed wind is in sharp contrast to the much quieter atmosphere surrounding it, and it is at its edges, and also just above it and below, where the wind speed drops off very sharply, that CAT is very frequently encountered.