You may hear about the jet stream on your nightly weathercast, but do you know what it is? According to the Glossary of Meteorology, a jet stream is defined as "relatively strong winds concentrated within a narrow stream of the atmosphere." What does that mean and how does it develop?

The jet stream plays an essential role in our weather. The discovery of it can be traced back to World War II by pilots who encountered it with high altitude aircraft. While the existence of the jet stream was suspected before this time, these high-altitude flights confirmed its reality.

Although the equator is heated by the sun nearly year round, the poles stay cold due to the lack of sunlight. This causes a temperature gradient between these locations; the stronger this gradient, the stronger the jet stream. The jet stream is usually strongest during the winter months when the temperature gradient is more extreme and the weakest during the summer.

Positions of the Polar and Subtropical Jet Streams

The above map shows the typical positions of the mid-latitude jet stream and the subtropical jet stream. During the winter, the jet stream typically shifts further south whereas during the summer it retreats northward. Also, the troughs and ridges are not as deep in the summer as they are during the winter. The jet stream meanders around and occasionally splits and reforms, however, the position of it is dependent upon cold air moving southward and warm air moving northward. This movement is also responsible for the development of troughs and ridges, which are sometimes called Rossby waves. The jet stream plays a significant role when forecasting the weather.

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