Jet streams can be located in the atmospheres of some planets which include the earth, and they are narrow and fast flowing air currents. The path of a jet stream tends to adopt a meandering shape as the current can be observed swinging from side to side. These streams are dynamic in their motion and may begin, end, break into multiple parts, merge into a single stream, or course in different directions. The common jet streams on earth are found near the altitude of the tropopause (the upper boundary of the troposphere) and flow from west to east. Two main types of jet streams can be observed, the weaker subtropical jets and polar jets. Both the Northern and Southern Hemispheres each have a subtropical jet and a polar jet. Typically, a jet stream could a few hundred miles wide and less than three miles in vertical thickness.
Polar jets are found in an altitude of 30,000 to 39,000 feet and between 30° and 60° latitudes, and being closer to 60°. They are formed when air masses of varying temperatures encounter, and the ensuing pressure difference causes strong winds. The resulting wind does not drift instantly from the cold region to the hot region, but is diverted and flows along the boundary of the two air masses. The speed of the wind is highest where the temperature difference is greater and usually surpasses 60 miles per hour. The polar jets often intrude into mid-latitudes and strongly affect aviation and weather patterns due to their low altitude. In the Northern Hemisphere, the polar jet moves over Asia, Europe, and North America and the oceans in between. In the Southern Hemisphere, they most often circulate around Antarctica throughout the year and gradually migrate northwards as the hemisphere warms and heads south as it cools. In the north, the polar jet stream is more is stronger, has a lower altitude, and covers many countries in the northern hemisphere.
Subtropical jets tend to have a higher altitude of 33,000 to 52,000 feet but are also weaker, and they are found at around 30° latitude. They are formed as a result of separating large air masses. This type of jet streams separates tropical air from mid-latitudinal air, and therefore their speed and temperature are not as extreme as polar jets because the temperature difference between the air masses is not as extreme. These jets affect precipitation and cloud cover in the regions it flows over.
The study of jet streams has aided professionals in both meteorology and aviation. In meteorology, it is now understood that jet streams affect storm systems at lower atmospheric altitudes levels. Therefore, the study of their flow is now a significant part of weather forecasting. In aviation, it has been discovered that flying at the altitude and in the direction of the jet stream makes for faster and consequently cheaper flying. The opposite also applies when flying against the jet stream and therefore helps airline plan their flights accordingly. Jet streams are also associated with a phenomenon known as clear-air turbulence which is a result of the wind shear caused by jet streams. This phenomenon can cause aircraft to plunge, and therefore pilots have to factor it during flights to avoid fatal accidents.