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One of the most powerful influences on weather, the jet stream is a band of winds that blow from west to east around the world at speeds up to 310 miles per hour, and between 4 1/2 and 7 1/2 miles above the Earth. Dramatic temperature differences between the polar and temperate regions generate the strong winds of the jet stream, which can meander as far north as Alaska and as far south as the southern United States.

The jet stream helps determine the location and movement of high-pressure and low-pressure systems and weather fronts; in effect, it steers storm systems as they move along in the lower atmosphere, said meteorologist David Miskus of the National Weather Service in Silver Spring, Md.Generally, the high-altitude jet stream moves from west to east because of forces associated with the Earth's rotation, said Miskus, but at some altitudes and under certain circumstances, the winds may blow in the opposite direction.

The stream's location strongly influences weather over a wide area. In addition, when the jet stream departs from its normal position for extended periods of time, it can cause abnormal stretches of cold, warm, dry or wet weather.

How do weather forecasters use it? With computerized mathematical models exist, Miskus said, they "can feed in current information, and the the model will predict the position of the jet stream" up to 36 or 48 hours later.

- Richard Saltus