The Northern Hemisphere Jet Stream can be seen crossing Cape Breton Island in the Maritime Provinces of Eastern Canada. The Jet Stream is a narrow zone of high-speed winds typically found at altitudes of 4 to 8 miles (8-12 km) above the earth. They result from temperature contrasts between polar and tropical regions. The strongest Jet Stream winds are found in the winter when the contrast between polar and tropical regions is the greatest. Wind speeds can reach 90 to over 180 miles per hour (145 to over 290 km/h) from west to east. During the winter months over the United States and southern Canada, the path taken by the Jet Stream can have a large influence on the weather conditions of this region. Photographed from the Space Shuttle Discovery during mission STS-39 in May 1991.