It seems like just about everybody has heard about the jet stream, but just what is it, and what does it have to do with the weather? The jet stream is simply a thin, narrow, band of very strong winds in the upper atmosphere. Although it is pretty rare for winds at the surface to reach 100 miles per hour, winds in the upper atmosphere reach 100 to 200 miles per hour just about every day. Interestingly, these strong winds are not found everywhere, but tend to be concentrated in very narrow, but long "streams." For example, suppose a jet stream with winds blowing from the west at 150 miles per hour is directly above Missoula. The "narrowness" of the jet stream may mean that at the same time and altitude, the winds over Kalispell, or Boise will be only 40 miles an hour. At the same time the jet stream may stretch from Seattle to Minneapolis. In the realm of the large "ocean" of air that makes up the atmosphere, these small zones of strong winds resemble "streams" of fast-moving water. The jet stream is also very "thin." Winds may be 150 miles per hour at 25000 feet, but only 50 miles an hour at 20000 feet and 30000 feet.
There tend to be two jet streams of strong winds between the equator and the North Pole. One is called the "subtropical" jet and the other, often located near the Idaho/Montana area, is called the "polar" jet. These streams tend to blow from west to east, but they can also make large detours south into the tropics or north into the arctic. Knowing the position of the jet stream is very important for jumbo jets that can fly at the high altitudes where the jet streams are typically located. Planes traveling from west to east try to get into the jet stream, to take advantage of the strong "tail wind." Planes traveling from east to west try to avoid the jet stream, since the strong "head wind" can slow their progress and cut fuel economy.
The jet stream is not just important for pilots. It is also very important for forecasting the weather that happens down here on the surface of the earth. The underlying reason for the jet streams in the upper atmosphere is the temperature difference between the warm tropics and cold Arctic regions of the earth. This temperature difference tends to concentrate in small zones called fronts, and above the fronts lie the jet streams. Along these fronts with large temperature contrasts, storms tend to grow from small disturbances into severe storms and blizzards. Since the jet streams lie above the fronts, the storms that form along the fronts are moved around by the strong upper-level winds. In essence, the jet streams tend to "steer" the storms. When the jet stream is over your area, strong storms may move by every few days, but when the jet stream is detouring around your area, long periods of calm and dry weather will tend to prevail.
The polar jet stream tends to be north of the Montana/ Idaho area in the summer and often south of the area during the deepest parts of the winter. During the spring and fall, the jet stream is often crossing Montana and Idaho, bringing periods of storms and very active weather.
- Tim Barker, Boise Science and Operations Officer