What is the jet stream?

The jet stream is a river of fast moving air high in the atmosphere. It's normally near the boundary between the troposphere and the stratosphere (tropopause). It normally blows at speeds between 110 and 140 knots. The jet stream normally is situated in the middle latitudes. During the winter months, it usually blows across the U.S., undulating back and forth. During the summer, it lifts north well into Canada. Once in a while, the jet stream does unexpected things like dive far to the south in the spring bringing unseasonably bitter cold to parts of the U.S. On rare occasions, it can go far to the north during the winter, causing mild winter weather in the U.S. The jet stream can normally be found between the tropical air masses and the polar air masses. The jet stream is responsible for causing mid-latitude storms to intensify rapidly. It can also cause spring and summer thunderstorms to grow stronger and severe. The jet stream is responsible for moving weather systems along. Jet airliners can take advantage of the jet stream to travel faster west to east such as a flight from New York to London. They try to avoid it when they are going the opposite direction.