The jet streams are belts of high speed winds occurring at about 5 to 10 miles above the earth and may occur at higher or lower levels. These winds blow at speeds of 150 to 300 miles per hour. Speeds to 400 mph have been reported. The streams of high winds blow from west to east about the Northern Hemisphere. They may be 2,000 to 3,000 miles long. 300 to 400 miles wide and 10,000 to 20,000 feet thick as they serpentine around the globe. The jet streams were first encountered by military aircraft during World War II, but prior to that they were suspected to exist due to the observations of fast moving high cirrus clouds. Pilots flying west fly to avoid them, when flying east will try to fly along with the jet stream to get an added boost of speed. The jet stream moves from side to side around the earth wandering north and south depending on the season. In winter in the Northern Hemisphere they move south to blow hardest at about 30 to 35 degrees north latitude or over the United States area. In summertime the core of the jet stream lies about 40 to 45 degrees north latitude. Similar jet stream movements also appear in the Southern Hemisphere. Mush has been learned about the jet streams, but an exact explanation for their origin is still lacking. One theory is that the jet streams may form by coming together of larger streams of equatorial and polar air masses at great heights. When these cold and warm air masses are side by side, there is a resulting difference of pressure and temperature and may cause a strong circulation which becomes a jet. In North America, the North Atlantic and Europe, observations have been sufficient to detect jets and forecast their behavior. The jet streams are closely related to the polar front which separates tropical and polar air masses at about 60 degrees north latitude. The polar front is a continuous breeder of weather of the temperate zones. The jet stream seems to play an important part in the formation, steering and intensifying of weather phenomena such as cyclones, hurricane tracks and abnormal weather spells.