Nearly all forms of weather, including Earth's most powerful winds, can be attributed to the fact that the Sun warms our planet unevenly. Because of the angle at which solar radiation strikes Earth's surface, regions near the equator are heated more intensely than those near the poles. This results in areas of relatively warm air near the equator and areas of relatively cool air near the poles. Warm air is less dense than cold air, and these density differences create air pressure differences from region to region.

Air moves horizontally from areas of higher pressure to areas of lower pressure. This movement creates wind. The greater the pressure difference, the stronger the wind blows. Moving upward into the atmosphere, the air thins, and the pressure difference between warm and cold air increases. Thus, the fastest winds occur at high altitudes where pressure differences are greatest, rather than at Earth's surface. These high altitudes are where the high-speed jet streams travel. In addition, air flowing towards areas of lower pressure is deflected toward the east by Earth's rotation. This deflection affects wind direction, so that air generally moves from west to east around Earth, as seen in the flow of jet streams.

One of the most extreme temperature and pressure gradients is found at the polar front, where consistently cold polar air meets consistently warm sub-tropical air. This boundary of differences gives rise to the polar front jet stream, a high-speed band of wind traveling at up to 400 kilometers per hour (249 miles per hour) that encircles Earth at altitudes of 10-15 kilometers (6-9 miles). It is located at latitudes of approximately 40-60º N and S. Sometimes it meanders wildly southward or northward, but in the northern hemisphere generally it moves from west to east. The jet stream moves in sweeping loops thousands of kilometers long, hundreds of kilometers wide, and two to three kilometers (one to two miles) thick.

The polar front jet stream is instrumental in both creating and moving storms over the United States. The fastest winds within a jet stream, called jet streaks, can induce horizontal pressure differences at the surface that initiate mid-latitude storms. These storms track across the country guided by the jet stream.

In the animation on the next page, you will learn more about this powerful phenomenon.

Source: Teachers' Domain, Giving Rise to the Jet Stream, published December 17, 2005, retrieved on November 18, 2010, http://www.teachersdomain.org/resource/ess05.sci.ess.watcyc.risejet/