A jet is defined as a narrow stream of rela-tively strong winds. On upper-level charts two such jets are often depicted. Polar-front jets are the most predominant and are associated with polar fronts of middle and subpolar latitudes. Like polar fronts, these jets can vary greatly in position and strength from day to day. The other jet that is often distinguishable is the subtropical jet. It is located at the poleward sides of the tropics, between 20 and 30 degrees latitude. To be classified as a jet, the wind speeds must equal or exceed 50 knots. However, an observation of high wind speed does not by itself warrant use of the term jet stream. As the word stream implies, the core must possess considerable length. The WMO defines it as normally thousands of kilometers in length, hundreds of kilometers in width, and some kilometers in depth.

The principal jet axis is marked by a heavy red line, with arrowheads indicating the flow direction. Secondary jets and jet fingers are depicted using dashed red lines with arrowheads. The jet axis, like isotachs, follows height contours. At 500 mb, it is located within 60 meters (plus or minus) of the 5,610-meter contour. At 300 mb, look for it between the 8,960- and 9,240-meter contours.

The polar-front jet lies vertically above the maximum temperature gradient of the middle troposphere. Because the 500-mb chart is repre-sentative of the mid troposphere, locate the maximum temperature gradient (isotherm packing) on this chart. The jet is normally found above and just south of this thermal band. This isotherm concentration is most often associated with a polar front, and the jet axis at 500 mb seems to coincide with the 17C isotherm on the warm side of the maximum temperature gradient.

The jet fluctuates vertically (up and down) and latitudinally (north and south). It usually lies between the 300- and 200-mb levels. It is nearer the 200-mb level at lower latitudes and 300 mb at higher latitudes. It also fluctuates seasonally. It is at a lower altitude in winter (300 mb) than in summer (200 mb).

The 300- and 200-mb charts give the best representation of the jet, but it is often well defined down at the 500-mb level. This level is very important in analyzing the jet, because it normally provides much better coverage (reports).

Upper-level charts allow us to locate jet streams and see their width and length. Another aid in locating their position is satellite pictures. The satellites play a key role over the oceans where upper level reports are very sparse. The combination of upper-level- wind observations and satellite pictures has given us a much better understanding of the various jet streams that exist within the atmosphere. There will be more on jet streams later in this chapter.

In summary, the isotach and jet stream analyses go hand in hand. Isotach patterns from 500 to 200 mb outline centers of wind speed maxima and their associated jet streams. The jet streams are rivers of fast-flowing air (50 knots or more) and extend for thousands of miles around Earth, are hundreds of miles wide, and have a variable depth of only a few miles. The jet core is usually located between 300 and 200 mb, but the entire stream fluctuates vertically and hori-zontally.

A combination of upper-level-wind reports and satellite pictures provides us with a means of locating the jets occurring within our atmosphere.

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