Five or six miles above our heads, the atmosphere is in perpetual motion, comprising a more or less leisurely drift of air from west to east.
Here and there, however, like Tennyson's brook, it "makes a sudden sally"; like a normally placid river that is forced to rush at high speed down a narrow channel, the flow constricts in places into a slender invisible tube of high-speed winds - the jet stream.
The name jet stream has nothing specifically to do with jet aircraft, but is intended to compare the atmospheric structure in the vicinity to a powerful jet of air. It so happens, however, that the jet stream can occur anywhere between 25-40,000 feet above the ground, which coincides with the levels at which airlines like to fly their jet-engined aircraft.
It typically blows at a speed of 100-150 m.p.h.; as a following wind, the jet stream can push a transatlantic aircraft to fuel economies worth many hundreds of pounds a flight, while contrariwise, as an unexpected headwind, it can lose an aircraft similar sums.
It was Dauntless Dotty, allegedly, who discovered the existence of the jet stream 55 years ago today, on November 24th, 1944. Dotty, apparently, was the lead aircraft of a group of B29 bombers which took off westwards from the US on the first high-level bombing mission to Japan in the closing months of the second World War. The flight encountered unexpected head-winds of unprecedented strength, which proved to be a more formidable adversary than the enemy's anti-aircraft guns and fighter aircraft; Dotty was the first to experience the unexpected jet stream on the nose.
US meteorologists, unlike their European counterparts, like to make a feature of the jet stream in their forecasts. No television presentation is complete without a shot of a broad red wavy line meandering across the Great Plains, or further north or south, to indicate its latest known position.
It is a useful tool to convey a pictorial impression of the general character of the weather over a large sub-continent like the United States.
Its usefulness is twofold. Firstly, since it indicated the approximate position of the polar front, it also delineates the average track of the most vigorous depressions on the weather map, and therefore the zone in which the most active weather is likely to be concentrated.
Secondly, and for the same reason, its meandering path provides a convenient dividing line between the cold polar air to the north, in which harsh wintry conditions are likely to be prevalent, and the much milder air of semi-tropical origin further south.