The Jet Stream

The jet stream covers the entire northern and southern hemispheres. It is a very fast moving current of air that causes global winds and climate change. The jet stream normally separates 2 areas that have different temperatures on either side.
ww2010.atmos.uiuc.edu

There are 2 separate streams in each hemisphere. There is the polar and subtropical jet streams. The polar lies closer to the poles of the earth and the subtropical is more near the equator.

www.srh.noaa.gov/jetstream/global/jet.htm

With each jet stream, there are many wind speeds within each one. In order to become a jet stream, the speed of the wind needs to be over 69 MPH. Normally the outermost layer of the main jet streams near around 80 MPH for wind, the inner comes closer to 140 MPH. This is because when the time of the year changes to different seasons, the jet stream moves with the sun and will move south in the winter to bring colder air in the winter, and warmer in the summer.

www.srh.noaa.gov/jetstream/global/jet.htm

The jet stream goes from West to East because of the direction of the Earth's spin. Because the Earth's axis is just slightly off, the way the jet stream also shifts to incorporate for the tilt.