The jet streams are defined as swift geostrophic air streams in the upper troposphere that meanders in relatively narrow belts. These are the strong cores of upper-level westerly winds which follow a meandering path. They follow the same course as the high-level westerlies.

It was towards the end of World War II that the existence of jet streams in the upper troposphere was made known to the meteorologists. The sequence of events leading to the knowledge of jet stream is quite interesting.

When during the last phase of Second World War the American bomber pilots tried to fly towards Japan at an altitude of about 13,000 meters, they encountered strong head winds which greatly reduced their ground speed (sometimes to zero).

But while returning to their bases in the east, they found that the speed became much faster and at times, it even doubled because of a high velocity tail wind.

ADVERTISEMENTS:

Thus, the pilots returning home from high-level missions brought back their unique experiences of the upper-level winds blowing with terrific speed. Ultimately the formal discovery of the so called ‘jet stream’ was made.

However, it is believed that there were certain clues to the existence of the jet stream as early as 1904. The early investigators while studying the high-speed cirrus clouds pointed out the existence of high velocity winds in upper troposphere. This fact was also ascertained by the studies of balloon ascents in the early twenties and thirties.

The term ‘jet stream’ was first applied to the high velocity upper-level winds during World War II. Since then they are becoming increasingly important to persons interested in upper-air currents and their possible impact upon the surface weather conditions. They have assumed still greater significance to high-flying aeroplanes.

According to Trewartha, the jet streams are relatively narrow bands of stronger winds bounded by slower moving air. Jet stream is also described as a westerly air current in the form of a flattened narrow core or tube, thousands of kilometers in length, a few hundred kilometers in width, and two or more kilometers in vertical thickness.

ADVERTISEMENTS:

According to Petterssen, the jet stream is almost entirely a thermal wind and its strength is proportional to the temperature contrast through the whole layer below.

Because of the steep north-south temperature gradients, the north-south pressure gradient increases with height up to the level of the jet. This is the basic reason for the existence of jet stream.

The westerly jet streams occur at elevations from 9,000 to 12,000 meters in the lower middle-latitudes. But at higher latitudes, say, 60° latitude, they are found at lower levels. They are circumpolar in character.

However, the jet streams do not always blow from due west, but have’ some pole-ward or equator-ward component. They appear as northwest wind at some longitudes, and as southwest wind at others.

ADVERTISEMENTS:

In other words, like the upper air westerlies of which they forma part, the jet streams of the multitudes follow a wavy and irregular path while moving around the earth.

Deviations in the path of jet streams are caused partly by the disturbing influence of continents, particularly in the northern hemisphere, and partly by the large travelling centers of high and low pressure (cyclones and anticyclones) at the sea level.

Jet streams are characterized by a great seasonal variation. During the colder part of the year they migrate towards the equator and their velocity also increases. In summer the wind speeds in jet streams are reduced to about half of what it is in winter because of smaller horizontal temperature gradients. In winter, the jet streams extend far into the tropics.

According to Franklyn and Cole, such seasonal variations are linked with the seasonal changes in atmospheric temperature at lower levels of the troposphere. The mean velocity of the jet stream winds is about 144 km per hour.

ADVERTISEMENTS:

But at times, the wind velocity in the inner cores of the jet stream may be at high as 480 km per hour or slightly more. During the cold season, generally the wind travels at a speed of 160 to 240 km per hour at the core of the jet stream.

Meteorologist assigns the name Polar-Front Jet to the primary jet stream. The Subtropical Jet is one which is found in the lower latitudes at a mean height of about 12 km. The Polar-Front Jet has the maximum speeds averaging 215 km per hour.

In extreme cases it becomes double of the average speed. Sometimes the Polar-Front Jet encircles the entire globe. There are diurnal variations in the position and speed of jet streams. At higher latitudes, the wind velocity is relatively higher on the right than on the left of the jet core.

On the contrary, at lower latitudes the wind to the right of the jet core is relatively slower than that on its left. Polar-Front Jet is always associated with the polar front, while the Subtropical Jet does not seem to have any relationship with any frontal zone.

ADVERTISEMENTS:

On occasions the jet stream breaks through the tropopause and enters into the lower stratosphere. At times, the jet stream effect extends down to an altitude of about 3 km from the earth’s surface. The stratosphere, in the absence of water vapour, remains practically dry and cloudless.

But whenever there is a break or discontinuity in the tropopause caused by infiltration of the jet, certain amount of water vapour manages to reach the lower parts of the stratosphere. Under these circumstances, the lower part of the stratosphere exhibits occasional cirrus clouds.

In the northern hemisphere, wherever the wind velocity is in excess of the pressure gradient, the high velocity wind shifts to its right and is called the sub-gradient wind. Because of the piling up of air to the right side of the jet stream, there is a high pressure area on the earth’s surface towards the equator-ward margin of westerlies.

This process is called ‘anticyclogenesis’. Since there is a marked variation in the wind velocity in the meandering jet streams in the upper troposphere, it makes weather prediction all the more difficult.

ADVERTISEMENTS:

Another peculiarity of the jet stream is found in the variation of its wind speeds between different longitudes. As stated earlier, there is a well marked longitudinal variation in the strength of the jet stream. It winter, the highest wind velocities of the jet stream are found near the east coast of Asia.

The jet is weakest over the eastern Atlantic and Pacific Oceans. This is dependent on the temperature contrasts. In summer, the picture is entirely different. Now, the strongest jet is positioned along the Canadian border. The Mediterranean Sea is another region over which the jet stream is very strong.

It may be noted that similar jet streams are found in the upper troposphere of the southern hemisphere. But the only difference is that because of the absence of huge continental areas in the sub-polar regions the southern hemisphere the jet stream is relatively more symmetrical than that of the northern hemisphere.

It shows that although the general flow of jet streams is almost parallel to the parallels of latitude, on occasions great meanders extending from north to south are also formed. That is why the isobars in a jet stream are wavy.

According to latitudes as well as altitudes there are giant size ridges of high pressure and troughs of low pressure. Even the smallest of the waves in the jet stream averages 6400 km in length. Its wavelength is also about 880 km.

These meanders are so large in size that the entire globe can accommodate only seven such waves. As these waves travel from west the east, they undergo significant changes in their size and extent.

They become progressively weaker too. Most of the waves, when fully developed, extend far into the tropics whereby the exchange of warm and cold air masses is made possible.

At a certain stage the amplitude of jet stream waves is increased to such an extent that the large pools of warm tropical air are cut off from the main stream and become isolated in higher latitudes. On the contrary, similar vast pools of cold polar air are cut off and isolated in lower latitudes.

It is in this way that the high-level cold cyclones and warm anticyclones come into being. It also helps in maintaining the latitudinal heat balance.

According to Trewartha, such an agitated circulation pattern brings about topsy-turvy weather on the earth’s surface. In fact, it is one of nature’s mechanisms whereby mass exchange of air between higher and lower latitudes takes place.