The jet stream is a current of fast moving air found in the upper levels of the troposphere which blow from west to east due to the Earth's rotation. This rapid current is typically thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Jet streams are usually found somewhere between 10-15 km (6-9 miles) above the Earth's surface. Wind speed within jet streams can range from between 50 -and 300 miles per hour. The position of the jet stream denotes the location of the strongest temperature contrasts between different latitudes on the Earth surface. Consequently, the strongest jet streams usually occur during the winter months, when large temperature differences exist between low and high latitudes. There are two main jet streams - the subtropical jet stream at about 30 degrees latitude and the polar jet stream at about 60 degrees latitude.