A relatively strong upper-level winds concentrated in narrow bands in the atmosphere. Jet streams are sometimes referred to as rivers of fast-flowing air and generally tend to flow from west to east.

Jet streams were first encountered by high flying military aircraft during World War II when American fighter pilots noticed that they burned much less fuel on their flights back from Japan.

There are two types of jet streams:
  • In the northern hemisphere, the polar jet stream is the boundary between the colder air that lies to the north and the warmer air to the south. In the winter, the polar jet stream movs south so that the leading edge of the cold air may extend into the southern United States, southern Europe and even south of Japan. In the summer, the polar jet stream is much weaker and is found no further south than Canada.
  • The subtropical jet stream which blows at an altitude of about 10 kilometers (33,000 feet) at 30 North latitude and is the cause of severe thunderstorms.