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A Jet Stream Runs Through It

© Keith C. Heidorn

High above the Earth's surface, rivers of air rush their way around the globe in a high speed current that often spawns side eddies that we on the ground call cyclones. Knowledge of the position of these streams, known as the jet stream because of their speed, is vital to accurate general weather forecasts and severe storm predictions. Knowledge of the location of the jet stream is also important to airlines and other long-distant air transportation.

In North America, the jet stream is a common topic for discussion on various weather media reports, but they often give us the impression there is just one such stream. In fact, both hemispheres have two major jet streams (which may sometimes branch): the polar jet stream and the subtropical jet stream. On occasion, a fast-moving current in the lower atmosphere may also, and this is known as a low-level jet.

The jet stream is generally defined to be a current of fast-flowing air at high altitudes, somewhere between 8-15 km (25,000 to 48,000 ft or about the 40-20 hPa/400-200 mb pressure level) above the Earth's surface. Jet stream wind speeds blow, by definition, in excess of 94 km/h (57 mph) and can reach nearly 500 km/h (300 mph). The jets are quite variable in their properties but a typical jet stream is hundreds to thousands of kilometres (miles) in length, about 160 to 500 km (100-300 miles) wide and about a kilometre deep (5/8 mile). (Though the polar jet streams form along the polar front in the Prevailing Westerlies belt that circles the planet, they do not always circumnavigate the globe as a continual stream.) The jet winds usually have a west to east direction, though they may loop with large north-south deflections.

I will only touch briefly on the subtropical jet stream here and focus more on its more influential (to residents of most of the US and Canada) sibling, the polar jet stream. The subtropical jet stream tends to form during the hemisphere's cold season. It is initiated by warm air flowing away from the tropical regions. This poleward air flow, however, is steered off its north-south course by the Coriolis effect until it takes a mostly easterly path at around 30 degrees latitude at an altitude of about 15 km (48,000 ft). The subtropical jet stream may aid in the development and steering of tropical storms and disturbances.

The Polar Jet Stream

The northern polar jet stream (also called the polar jet, or the mid-latitude jet stream, or just the jet stream) is the one having the most influence on weather across much of the United States and Canada, and thus it gets the most attention on local and national weathercasts. The polar jet stream can usually be found somewhere in the latitude belt from 40-60 degrees at an altitude from 7600 metres to 10,600 m (25,000-35,000 ft).

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