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A Jet Stream Runs Through It - Page 2

© Keith C. Heidorn

The highest wind velocity is found in the jet stream core where speeds can be as high as 460 km/h (290 mph) in the winter. The jet stream core region averages 160 km/h (100 mph) in winter and 80 km/h (50 mph) in summer. Those segments within the jet stream where winds attain their highest speeds are commonly known as jet streaks.

The polar jet stream position marks the location of the strongest temperature contrasts between polar and subtropical latitudes on the Earth surface. Therefore, the strongest polar jet stream velocities usually occur during the winter months. During the summer months, when the equator-pole surface temperature differences are less dramatic, the jet winds blow slower and are usually found at higher latitudes.

Jet Stream Formation

The polar jet stream is formed in the region of greatest contrast between polar and subtropical air known as the polar front where the cold, dry polar air meets the warm, moist air from the subtropical regions. From a climatological viewpoint, the position of the polar front forms a more or less even band around the globe, slipping north and south with the seasonal changes.

In the vicinity of the polar front, the air pressure drops more rapidly with increasing altitude in the denser cold air than in the less dense warm air. This temperature effect on air density results in air pressure at any given altitude being higher on the warm (equatorward) side of the polar front than on the cold (poleward) side. When cold air and warm air masses sit side by side, the higher the altitude, the greater the pressure difference between the cold and warm air. Thus, across the polar front, the horizontal pressure differential (or gradient) causes air to flow from the warm side of the front towards the cold side.

Once the air begins to flow, it is deflected by the Earth's rotation (called the Coriolis effect) and prevented from flowing directly from high to low pressure. Air flowing from higher pressure towards lower pressure is deflected to the right in the Northern Hemisphere (or to the left in the Southern Hemisphere). The resulting flow produces a westerly wind current generally flowing towards the east, parallel to and above the polar front. Very strong temperature and pressure gradients in the polar frontal zone can intensify these wind speeds to over 94 km/h, thus forming a jet stream.

Polar Jet Stream separates cold polar air from warm tropical air.

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