Cinema has many memorable lines. Who can forget, for example, Ingrid Bergman's "Play it, Sam!" in Casablanca, or the many times Michael Caine has told us "Not a lot of people know that"? And similarly enduring is Bette Davis's injunction in All About Eve: "Fasten your seat-belts. It's going to be a bumpy night!". Few seasoned air travellers, indeed, can have failed from time to time to "hit an airpocket", as it used to be described. The phrase, which nowadays is rarely used, conjures up images of great celestial bubbles of invisible void floating in the atmosphere, into which, when least expecting to, the unwary aeronaut may fall. The reality, however, is somewhat less bizarre.
The phenomenon is properly known as CAT - or Clear Air Turbulence. It occurs mainly at high altitudes, and results from very sharp changes in wind strength or direction over short distances, either in the vertical or horizontal. The result can be a violent buffeting as spectacular as might be expected in association with the severest thunderstorm.
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.
CAT, unfortunately, is very difficult to predict with confidence. It is also ephemeral in nature; it often happens that one aeroplane may hit a turbulent zone quite unexpectedly, while a following aircraft on exactly the same track may experience no turbulence at all. 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 is likely to rise significantly above this figure. They look most closely at the vicinity of the jet stream.
High above our heads most of the atmosphere drifts gently more or less from west to east. But here and there this eastward motion takes on a sudden burst of urgency, and the air is funnelled, as it were, into a very strong, concentrated tube of wind surging forward at 100 or 150 miles per hour. This jet meanders; sometimes it is relatively straight, but very often it undulates in a wavy, U-shaped pattern, and may lie anywhere between 25 and 40 thousand feet above the ground. Wind speed drops off sharply at its edge, and also above it and below, providing a sharp contrast with the much quieter atmosphere around, and it is here, "near to the river's trembling edge", that CAT may often be experienced.