How the jet stream affects an aircraft?

Do you know what a jet stream is?

Jet Stream is defined as a flat tubular current of air, quasi-horizontal, whose axis is along a line of maximum speed and which is characterised not only by great speeds but also by strong transverse (horizontal and vertical) gradients of speed (World Meteorological Organisation).

It is a strong current of air thousands of miles long, hundreds of miles wide , and several miles deep. Jet streams are found in the upper atmosphere, usually above 32,000 feet.

Meteorologists (scientists who study weather and climate in the atmosphere) know that jet streams can travel at hundreds of miles an hour. They were discovered toward the end of World War II by American bomber pilots over Japan and by German reconnaissance aircraft over the Mediterranean.

Meterologists also know that there are two main causes of jet streams:

The first main cause is that the hot air of the tropics rises and moves from the equator toward the North and South Poles. The warm air is pulled toward the cold air just the way steam always rises.

The second main cause is that while warm air is moving north and south to the poles from the tropics, the earth is also rotating on its axis from west to east. This west-to-east circulation of the earth creates a gigantic cross-current motion that ''bumps'' into the north-south moving air. It is at the bumping points where the jet streams begin.

Because of their strong winds, jet streams play an important role in the economy of the aviation industry. When an airplane flies into a jet stream for any period of time, extra fuel must be used. Not only is the fuel expensive, but it takes up room where either passengers or cargo could be. Just the opposite occurs when an airplane flies with a jet stream at its back. Fuel and money are saved because the wind pushes the plane along.

If you have ever flown in an airplane, you probably have experienced something called turbulence. Turbulence causes the bouncing, shaking feeling of the airplane. It means winds of different velocities (speeds) are buffeting the airplane. Besides a local weather storm, turbulence to an airplane may be caused by a plane flying into or out of a jet stream.

The turbulence caused by a jet stream occurs at the very edges of the jet stream, where the high-speed winds (over 200 miles per hour and sometimes all the way up to 300 m.p.h. over Japan) pass through the surrounding slower-moving air.

Just think of your plane as a canoe shooting down a river, only the water is wind in the sky. If a river is deep and straight, the water will run smooth and without waves even if the current is strong. Wherever the river bends and curves and the main currrent meets slower moving backwater rapids, or white water, occurs. This will cause your canoe to bounce around. This is exactly what happens to an airplane in the jet stream.

The wind currents along the edges of the jet stream are choppy or turbulent as high-speed air meets more slowly moving, nearly stationary air (about 20 m.p.h.). When a plane flies through this it bounces around. And since jet streams snake all over the sky, a plane can fly in an out of the turbulence many times in a single trip. To avoid turbulence pilots will fly to higher or lower altitudes to get beneath or above the jet stream.