A jet stream is a river of high-speed, high-altitude air thousands of miles long, a few hundred miles wide and a few miles thick flowing between six to nine miles above the earth’s surface.
There is a polar jet and a higher-altitude, slower-moving tropical jet in each of the Northern and Southern hemispheres around latitudes 30 degrees and 60 degrees. The winds of the jets travel from west to east near the tropopause, the interface between the troposphere and the stratosphere. Jet streams flow along the boundaries between continent-size air masses that generally coincide with the subtropical highs and the polar front, but a jet can break off into smaller rivers of air that might merge again downstream or fade away completely.
A jet stream is driven and controlled by two effects. One is a heat-driven pressure gradient force, which moves air from high to low temperature. The other is the Coriolis effect, which derives from the change in speed and direction of rotational vectors with latitude. Because air moves with little or no friction in the upper troposphere, it is extremely sensitive to the Coriolis force.
In the global circulation, the Coriolis effect combined with conservation of momentum causes three circulation cells to form. The troposphere is thicker in equatorial regions due to its higher temperature, so air flows downhill with the pressure gradient and heads poleward. As it moves, the Coriolis effect turns the flow eastward.
As the air moves towards the poles, it must conserve momentum, which means it must accelerate relative to the earth below. At higher latitudes the ground speed of rotation slows while the wind keeps the speed it had originally, thus increasing its speed relative to the ground.
By the time it reaches 30 degrees latitude, the air is moving eastward, and the pressure gradient is balanced by the Coriolis effect so it flows along the boundary of the subtropical high. Meanwhile, as air moves poleward along the surface after descending within the subtropical high, it encounters the greatest temperature gradient at the polar front, which drives the polar jet eastward at that boundary.
At the polar front the warm tropical and cool polar air masses have a large temperature differential, and the density difference is highest within the transition zone between the two air masses. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary in the polar jet.
In the polar jet stream, large meanders known as Rossby waves propagate eastward at speeds that are slower than the wind within the stream. Smaller short-wave packets of upper-level energy move through the flow pattern around long-wave ridges and troughs within Rossby wave.
With arctic temperatures rising, weaker temperature gradients produce weaker jet streams that tend to meander more, increasing the amplitude of the Rossby wave. The meanders tend to stay in place longer, bringing warm weather farther north and cold weather farther south.
The polar jet streams have a profound effect on global weather, and knowledge of their locations and movements has become an important part of weather forecasting.
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ON THE NET:
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Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
