Jet streams are narrow bands of strong westerly winds that form near the tropopause at the intersections of the Hadley, Ferrel, and Polar cells that make up the global atmospheric circulation. Each hemisphere has two main jet streams: the polar jet, which meanders around the globe at approximately 60°, and the suptropical jet, at approximately 30°.
Jet streams are usually a few hundred km wide and around 5 km thick. The winds are the strongest at the very center of the jet stream flow and range from 50 to as high as 220 kts. A 100 kt jet stream is considered to be strong.
The polar jet stream is stronger than the subtropical jet because the Coriolis force is stronger at higher latitudes. Furthermore, because the jet stream is fundamentally driven by the temperature gradient between the equator and the poles, it is stronger during the winter when the temperature gradient is the largest. The height of the tropopause varies with latitude, so the polar jet at (9–12 km) is lower than the subtropical jet (10–16 km). The height of the jet stream corresponds to a pressure level of approximately 250 hPa.
Jet streams mark the boundaries between polar air, the temperate air of the midlatitudes, and tropical air. They do not follow a straight path around the globe; instead, they tend to meander under the influence of lower level conditions. These meanders have the appearance of waves and are known as Rossby waves which move slowly westward around the planet. In some cases, disturbances to the jet stream flow can cause a section of the jet stream to branch or break off. On other occasions, the polar and subtropical jets may merge.
The jet stream and Rossby waves have a significant impact on global weather. For example, if the polar jet stream circulation is characterized by high amplitude Rossby waves, then this means that the jet stream wind has more of a north/south orientation. Large amplitude Rossby waves mean that polar air can travel a long way equatorward. Likewise, warmer air from the midlatitudes can extend a long way poleward. These fluctuations can result in highly unseasonal conditions weather conditions.
For example, the term polar vortex, used to describe very cold weather events in the northern hemisphere, is a manifestation of polar jet with high amplitude Rossby waves allowing the frigid air from the Polar cell to travel much further equatorward than usual.
The position of the jet streams are a good indicator of expected weather conditions. For example, in the mid latitudes, once the polar jet stream has migrated to higher latitudes, very cold winter conditions are less likely and more settled conditions are likely to prevail. Furthermore, the position of the subtropical jet can be used an indication of the expected extent of the trade winds. The position of the subtropical jet is also important in terms of hurricane formation and steering.
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