English: The jet stream is the comparatively narrow current of air that moves around the Northern and Southern Hemispheres of the Earth in wavelike patterns. It can be compared to a "river" of wind moving at high speed. The jet stream varies from about 100 to 400 miles (161 to 644 kilometers) wide and 1 to 3 miles (1.6 to 4.8 kilometers) thick. Its strongest winds are generally encountered at about 30,000 feet (9,144 meters) in altitude—in the troposphere. Jet-stream winds usually have a speed of 150 to 300 miles per hour (241 to 482 kilometers per hour), but speeds up to 450 miles per hour (724 kilometers per hour) have been recorded. Its general motion is from west to east. However, there also is a tropical easterly jet stream, which occurs during summer. This jet stream originates in the upper troposphere near Burma and extends to the west of Africa, some 6,000 miles (10,000 kilometers) to the west. The most extreme differences in temperatures occur where the stream is the narrowest.