A "wake vortex", as we noted yesterday in Weather Eye, is a spinning stream of air, a vertical whirlpool generated by an aircraft as it moves at high speed through the atmosphere. Usually two are generated, shed rearwards from the two wing-tips and rotating relative to each other in opposite directions.
At high altitude wake vortices are harmless. At ground level, however, the twin whirlpools twist and writhe, interacting with each other and with the ground below. The severe turbulence generated is potentially catastrophic when encountered by a fixed wing aircraft, and may well have been partially responsible for the recent American Airlines tragedy in New York.
But there are other kinds of turbulence, naturally occurring, with which aircraft also have to cope. Although they all have the same effect, making conditions uncomfortable and sometimes dangerous for passengers and crew alike, meteorologists like to categorise, and distinguish carefully between mechanical turbulence, thermal turbulence, and CAT.
Mechanical turbulence occurs in the first few hundred feet above the ground when a strong wind blows over a rough countryside. The effect is similar to that which occurs in water when a shallow river flows over a rocky bed and breaks up into choppy waves and eddies. Mechanical turbulence is rarely a very serious hazard, and in general it affects aircraft only when they are taking off or coming in to land.
Thermal turbulence, as its name implies, is caused by heat. When the sun warms the Earth, it happens that some pockets of air become warmer than others; these become buoyant, and generate rising currents which may at times - especially in summer - be very powerful. Downward currents then develop nearby to replace the ascending air. As an aircraft flies through such a region, these alternately rising and then falling zones of air are felt as turbulence. When the thermals, as they are called, are strong enough to build up into the mighty cumulonimbus clouds which give us thunderstorms, the turbulence can be severe.
Turbulence of the third kind is known as CAT, which stands for "clear air turbulence". This occurs high above the ground at cruising altitudes, often near the edge of the narrow band of very strong winds we call the "jet stream". It is normally encountered well above the levels where any clouds are present - hence its name - and it is also very difficult to predict with confidence. It is ephemeral in nature; often an aeroplane may hit a zone of CAT quite unexpectedly, while an aircraft on exactly the same track may have no turbulence at all.