What is the Jet Stream?
When you hear this term used by forecaster, they are referring to fast moving air currents found around 36,000 feet up (roughly 11,000 meters). These currents represent the boundaries between the cold and warm air. [1] Since the clash of warm and cold air is essentially the root of all weather, meteorologists use the jet stream in many ways. One of key ways is that the jet stream steers storm systems across continents. For example, my state of Wisconsin will get a lot of snow if the jet stream dips south over the central states and then steers the storm north towards Wisconsin. When the storm system is in warmer climates it will collect moisture (cold air can’t hold as much moisture) and then this moisture will hit the colder Wisconsin air resulting in a lot of snow…the rough appoximation is 1 inch of rain equals 10 inches of snow. That’s 1 cm rain to 10 cm snow for readers who live in countries smart enough to adopt the metric system…we Americans are a stubborn people! 
Also, you might here about jet streams from the captain on your next flight because the winds can either shorten or lengthen transit time for flights. In the summer, the jet stream moves an average of 55 km/h (~35 mph) and in the winter an average of 120 km/h (~55 mph). Jet streams have been measured as high as 400 kh/h (250 mph)….that’s a mighty wind that could make you arrive early (or miss your connecting flight!) [1]
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Posted by Tim Roth, author of the political blog Think Anew and Act Anew
Source:
1. “Jet Stream”, Wikipedia entry
[...] The driving force behind all the moisture and wind is the jet stream that is currently very dynamic. Below is the path of the current jet stream: [...]