CLIMATOLOGY. Climatic charts depicting jet stream winds are somewhat misleading. They show a single jet (figures 8-3-7 and 8-3-8), when

Figure 8-3-6.Jet fingers.


Figure 8-3-7.Mean seasonal jet stream distribution for January.


Figure 8-3-8.Mean seasonal jet stream distribution for July.

in fact there are more than one. This single or mean jet stream is the result of the averaging of all the upper-level westerlies over a given hemisphere for a specific period of time. The mean jet stream of the Northern Hemi-sphere is found between 25N and 45N, depending on the season of the year. The polar-front jet and subtropical jet both exist within these boundaries; however, the mean should not be thought of as being specifically representative of either. REMEMBER, THE MEAN JET IS DERIVED FROM ALL UPPER-LEVEL WESTERLIES OVER A GIVEN HEMI-SPHERE, FOR A GIVEN PERIOD.

Climatic statistics show that the mean jet of winter is stronger (70 knots) than the mean of summer (35 knots), that the jet core is slightly higher in winter, and that the depth of jet winds is much greater in winter. This last fact is the reason we use the 300-mb chart in jet stream analysis during the winter and the 200-mb chart during the summer.

No matter what the season, certain areas of the Northern Hemisphere have greater jet stream winds than others. The strongest jets are en-countered over Japan when a polar-front jet and a subtropical jet merge over the area. Winds close to 300 knots are not uncommon when this merger takes place. Other areas where strong jets are the norm are off the east coast of the United States, over the Sahara Desert, and the Arabian Sea. From this, we can say that higher velocity jet streams are concentrated near east coasts of continents, and lower velocity jets are found over west coasts of continents. There is also a greater latitudinal fluctuation in jet streams over the west coasts of continents.

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