The strongest winds in the atmosphere, you may recall from yesterday, are generally to be found some 35,000 ft above the ground, with a typical speed of perhaps 40 to 50 m.p.h.
But here and there, this more or less leisurely drift of air, like Tennyson's Brook , may "make a sudden sally"; like an otherwise placid river forced to rush at high speed down a narrow channel, the flow of air constricts in places to a slender invisible tube of high-speed winds - the jet stream.
The name is innocent of aircraft, but rather compares the atmosphere in the vicinity to a powerful jet of air. Very strong winds in the upper atmosphere had been observed in the 1930s using radiosonde balloons, and also noted in the motion of high cirrus clouds.
The term Strahlstroemung , or jet stream, to describe them was first used by German meteorologist Helmut Seilkopf in 1939, but their significance was only realised a few years later during the second World War when high-flying bomber aircraft began to report very strange occurrences.
In 1943, for example, a flight of British bombers ran out of fuel over occupied France when they encountered head winds of more than 230 m.p.h.
Then the Americans had several similar experiences when their B-29 aircraft faced even stronger winds over Japan in 1944. It was evident that Seilkopf's Strahlstroemung was indeed something to be reckoned with.
The Japanese military authorities were quick to see the potential of the jet stream as a weapon in the war, and in the closing stages of the conflict some 9,000 "balloon bombs" were launched from Japan, intended to carry incendiary devices across the 6,000 miles of the Pacific.
Only a few succeeded in their mission, but until the dynamics of the jet stream were fully understood the American authorities were puzzled as to how these missiles could arrive with such speed and such precision.
We know more about the jet stream now. It may be anywhere between 25,000 and 40,000 feet above the ground, and often meanders in a wavy U-shaped pattern. The narrow tube of strong winds is normally 100 miles or so in diameter, and typically blows at a speed of 100 to 150 miles per hour - although winds as strong as 300 m.p.h have been experienced near its core.
The slender river of swiftly moving air, in stark relief from the much quieter atmosphere surrounding it, is caused by a sharp contrast of temperature in the horizontal - a sudden lurch in the otherwise gradual decrease in temperature from the equator towards the pole.