Global Warming Web
Jet stream is a band of high-velocity atmospheric current that encircles the Earth. This band of strong winds is typically found in the upper troposphere and lower stratosphere. In both the Northern and the Southern Hemisphere, there are two distinctive jet streams: one is located just outside the tropical latitudes, in the subtropics; the other is located at the boundary of the midlatitudes and the polar region. In these latitudes, the jet-stream winds are westerlies, blowing from west to east. The two jet streams are named, respectively, “the subtropical jet” and “the polar jet.” These jets shift locations seasonally.
The rotation of the Earth around its own axis causes the air that surrounds the Earth to move as a result of the drag exerted by the Earth’s solid surface. This movement of air is called “wind.” Wind can blow in any direction, even though the Earth rotates from west to east. Two of the major forces in the atmosphere that determine wind direction are the pressure gradient force and the Coriolis effect. Because of differential solar heating between the tropics and a polar region, there is a strong tendency of the atmosphere to move outward from the warmer equator to the colder poles, distributing heat. Jet stream this tendency would cause winds to blow in the north-south directions in the Northern and Southern Hemispheres at different altitudes.
Owing to the Earth’s rotation, these two major north-south circulations break up, forming several smaller cells of circulation. These smaller cells include the Hadley circulation, the Ferrel circulation, and the polar circulation, listing them from the equator to the poles in each hemisphere. At upper atmospheric levels, when air travels to the north in the Northern Hemisphere, the Coriolis effect turns this airstream to its right, turning a southerly wind into a westerly wind. Furthermore, because the Coriolis effect becomes stronger as air travels further north and the effect of Earth’s surface drag is smaller at upper atmospheric levels, these westerly winds become very strong. As a result, jet streams form outside of the tropics, toward high latitudes, at upper atmospheric levels.