What determines the speed and direction of...

A jet stream is a narrow filament of air that streams from side to side, wandering north or south of the prevailing westerly wind with the seasons. Jet streams are normally found in the troposphere about six miles above Earth. Their speed can be up to 180 mph, depending on the season. During the winter, when there are greater temperature contrasts, the jets are stronger and shift toward the equator. In the summer, when temperatures are more uniform, they tend to weaken and shift toward the pole. This creates the “bends” in the jet stream portrayed by television meteorologists.