Figure 22 – jet streams: (a) maximum speed at centre; (b) Polar and subtropical jetstreams in both hemispheres; (c) cross-sectional view of jet streams

Applying this on a global scale by associating poles with cold air column and equator with warm air column, the gradual poleward decrease of temperature in the atmosphere from the equator should result in a large westerly component in the upper winds. It was found in 1940s during Second World War that high-flying aircraft encountered upper winds of very great velocity. These are known to be concentrated bands of rapid air movement, which are termed jet streams. Few of the features of jet streams are:

♤ These are narrow belts at the high altitude near the top of the troposphere.

♤ Their speed varies from about 110 km per hour (kmph) in the summer season to more than 180 kmph in the winter season.

♤ Their shape is circular. Speed in the jet streams decreases radially outwards (figure 22(a)). One way of visualizing this is to consider a river. The river's current is generally the strongest in the center with decreasing strength as one approaches the river's bank. It can be said that jet streams are "rivers of air".

♤ They are several hundred kilometers wide and about 2 km to 5km deep.

♤ The flow of jet streams is not in form of straight line. Their circulation path is wavy and meandering. These meandering winds are called Rossby waves

♤ They dip and rise in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else.

♤ Jet streams also "follow the sun" in that as the sun's elevation increases each day in the spring, the average latitude of the jet stream shifts poleward. (By Summer in the Northern Hemisphere, it is typically found near the U.S. Canadian border.) As autumn approaches and the sun's elevation decreases, the jet stream's average latitude moves toward the equator.

♤ On occasions the jet stream breaks through the tropopause and enters into the lower stratosphere. Certain amount of water vapour manages to reach in lower stratosphere with jet streams and this layer exhibits occasional cirus clouds. At times, the jet stream effect extends down to an altitude of about 3 km from the earth’s surface.

♤ There is a well marked longitudinal variation in the strength of the jet stream. In winter, the highest wind velocities of the jet stream are found near the east coast of Asia and weakest over the eastern Atlantic and Pacific Oceans. In summer, strongest jet is positioned along the Canadian border and Mediterranean region.

Two permanent jet stream zones occur in each hemisphere. One is sub-tropical jet stream and another is polar front jet stream. There is another jet stream which moves seasonally near equator. Description of these three streams is give below: