. Compendium of meteorology. Meteorology. TROPICAL CYCLONES 899 oceanic areas where upper-air data are lacking. Satis- factory forecasting of recurvature is impossible without accurate constant-pressure charts. Riehl and Shafer [14] studied some 57 tropical cyclones occurring be- tween 1935 and 1943 and classified them in groups as follows: (1) seventeen which curved north or north- eastward, (2) twenty-three which continued between west and north without recurvature, and (3) seventeen which had irregular tracks, particularly motion with a southward component for a portion of the track. It sho
. Compendium of meteorology. Meteorology. TROPICAL CYCLONES 899 oceanic areas where upper-air data are lacking. Satis- factory forecasting of recurvature is impossible without accurate constant-pressure charts. Riehl and Shafer [14] studied some 57 tropical cyclones occurring be- tween 1935 and 1943 and classified them in groups as follows: (1) seventeen which curved north or north- eastward, (2) twenty-three which continued between west and north without recurvature, and (3) seventeen which had irregular tracks, particularly motion with a southward component for a portion of the track. It should be noted that the "abnormal" storm tracks of the last group were as frequent as the "normal" tracks of the first group. Riehl and Shafer found that a tropical cyclone will begin to react when the forward edge of the westerlies— at least as low as 8000-14,000 ft—is found 500-700 miles to the west. As the polar trough advances east- ward, the storm turns from a westward to a northward path. The most difficult, though frequent, cases oc- curred when storms encountered strong troughs and an intrusion of the polar westerlies into the tropics similar to group (1). In that group, however, the base of the westerlies not only lowered but remained low in the subtropics and part of the tropics. In group (2), how- ever, the westerlies died away quickly to the rear of the polar troughs and the easterlies were re-established quickly and storms continued in a westerly direction. This condition developed when a warm high follows or develops to the rear of a polar trough and also if the polar trough advances from the west against a blocking high resulting in fracture. The tropical cyclone usually responds within 24 hr after the easterhes build back to 14,000 ft and above. Group (3), the "abnormal" tracks, result from a number of causes. A storm which has already completely or partially recurved may turn back northwestward or westward when blocked by a warm
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