The jet stream is a narrow ribbon of strong winds blowing in higher levels of the atmosphere. It’s winds are generally located at approximately 30,000 to 35,000 feet (9,000 to 10,500 m) above sea level. The speed of these strong jet stream winds can be as much as 300 mph during the winter (although it was measured at 400 mph over Scotland). These winds are caused by cold polar air colliding with warm tropical air in mid-latitudes. The greater the temperature difference between the polar and tropical air the greater the pressure difference and hence the stronger the winds of the jet stream.
Jet streams were first discovered by American pilots flying high on bombing raids above Japan during the second world war, although these winds had previously been observed from the ground years earlier. The American pilots found it impossible to accurately drop their bombs as they had been blown up to 12 miles beyond their anticipated target by the powerful wind. Jet streams are typically continuous over long distances, but discontinuities are common. There are two main types at polar latitudes and two minor subtropical types closer to the equatorial regions. The streams are most often found between the latitudes 30°N and 70°N for the polar jet stream and between latitudes 20°N and 50°N for the subtropical jet stream. There are other flows in the atmosphere that are referred to as jets, such as the equatorial easterly jet which occurs during Prostastream the Northern Hemisphere summer between 10°N and 20°N, and the nocturnal polar low level jet in the Great Plains of the USA. Occasionally, a jet stream can even split its flow or cut off into a closed circular flow. An associated jet stream phenomenon is known as clear air turbulence (CAT), caused by vertical and horizontal wind sheer connected to the streams.
The jet stream has important ramifications for airlines. For example in the United States and Canada the time needed to fly east across the continent can be decreased by approximately 30 minutes if an airplane can fly with the stream, or increased by more than that amount if it must fly west against it. On longer intercontinental flights, the difference is even greater; it is faster and cheaper to fly eastbound along with the stream and fly around the jet stream going westbound than it is to take the shorter, more direct, route between two locations. Meteorologists now understand that the path of the jet stream steers cyclonic storm systems at lower levels in the atmosphere, and so knowledge of their course has become an important part of weather forecasting. In 2007, Britain experienced severe flooding as a result of the polar jet staying south for a large portion of the summer.