I'm Dave Thurlow for the Mount Washington Observatory and this is The Weather Notebook. If there's one scapegoat that TV weatherpeople love, it's the jet stream. But there isn't just one jet stream, there is a whole family of jet streams for meteorologists to blame things on.
There are three main jet streams circling the globe in each hemisphere, about three to seven miles above the earth. Each hemisphere -- north and south -- has a polar jet stream and a subtropical jet stream. In between the latitude of each of these jets is the temperate zone of the midlatitudes. In the wintertime, the polar jet sags south across the U.S., reflecting the southern extent of the cold air below. But in summer, the polar jet retreats in to Canada and the subtropical jet arcs north to replace it, helping to trigger warm weather, violent thunderstorms and heavy rain. The subtropical jet made an extra early appearance in the Ohio River Valley just last month.
In the central U.S., there's still another player in this weather drama called the low-level jet stream. It's an intermittent but powerful conveyor belt of moisture that streams from the Gulf of Mexico up through the Midwest. This low-level jet is poised only about a mile above the ground, but it can race northward at close to 100 miles an hour, especially at night. The low-level jet was particularly active in the summer of 1993, when it helped to bring buckets of unwanted Gulf moisture to the flood-ravaged Mississippi. So, go ahead, if you have bad weather, blame it on one of the jet streams, you'll probably be right.
Funding for The Weather Notebook is provided by The National Science Foundation. Our show is underwritten by Subaru, maker of the all Weather Legacy. Subaru -- the beauty of All-Wheel Drive.