The Jet Stream

On November 4, 1944, ninety-four B-29 Superfortress bombers were approaching Japan on the first mass bombing assault of the war on Tokyo. At altitudes between 27,000 and 33,000 feet, the planes turned east over Mount Fuji and began their bombing run. Suddenly, the startled pilots found themselves roaring past landmarks at a speed of almost 450 miles per hour, about 90 miles per hour faster than the theoretical top speed of the aircraft. It was too late for most bombardiers to make adjustments, and most bombs fell miles beyond the intended target. Of the more than 1000 bombs dropped, only 48 fell anywhere near the objective.

From the military viewpoint, the mission was a dismal failure, but meteorologists were fascinated. What could explain a river of air at high altitudes that was ripping along at some 140 miles per hour?

After the war, high-altitude aircraft and balloons confirmed the existence of ribbons of rapidly moving-air, normally 180 to 300 miles wide and up to two miles thick. The average speed of these jet streams, as they came to be known, was between 60 and 115 miles per hour, and some with speeds in excess of 290 miles per hour were recorded. They were found in the subtropics and high in the stratosphere over the polar regions.

In the early 1950's, scientists at the University of Chicago devised a simple experiment to study the movement of air in the upper atmosphere. It consisted of an ordinary dishpan on a turntable which was cooled in the center (representing the poles) and warmed around the rim (representing the tropics). They poured in some water mixed with dye, and added some aluminum powder.

When the turntable was stationary, the metal particles moved upward from the heated edge and inward toward the cold center, where they sank and returned to the rim. This circulation pattern is what would be expected from a simple convection current.

However, when the pan was rotated, two interesting features were noted. Extending around the pan, halfway between the rim and the center, patterns of whorls and eddies appeared in the particles and dye. It is now known that these features correspond to the cyclones of the mid-latitudes, which do much of the work of transporting heat from the tropics to the poles.

The other unusual feature was a heavier concentration of streaks that looked like a kind of river winding its way around the pan and threading in between and around the zones of turbulence. This line marked a miniature jet stream.

In actuality, the most important jet streams, blowing from west to east, are located in the upper reaches of the polar front--the border between the temperate and polar air masses. In addition, there is one between the temperate and tropical air masses. Occasionally, the polar-front and subtropical streams merge, and their combined power can generate severe storms below.

There is also a jet stream called the polar night jet, so-called because they appear in the dark winter months high above the polar regions. During the months when a polar region is engulfed in frigid darkness, the air high over the poles becomes much colder than the air over the Equator. This difference in temperature gives rise to extreme air-pressure differences in the stratosphere which, when combined with the Coriolis effect, create the polar night jets which race eastward at altitudes of about 30 miles.

(Partially excerpted from Atmosphere by Time-Life Books).