A PRELIMINARY STUDY ON THE DETERMINATION OF NATURAL SYNOPTIC SUMMER SEASON OVER ASIATIC NATURAL SYNOPTIC REGION AND THE PREVAILING WEATHER PROCESS IN THIS SEASON

The synoptic process on 500mb over Asiatic natural synoptic region have been analized for 5 summers.The results are given below:1.The variation of the position and strength of the maximum westerly wind belt at 500mb surface along four meridians 65°E,105°E,120°E,140°E gives a good index in determing the natural synoptic seasons over Eastern Asiatic synoptic natural region.The disappearence of the low latitude westerly wind maximum along 65°E is the first indication of the beginning of Bai-Ü period.When the westerlies wind maximum belt to the south of 40°N completely disappears along 140°E summer begins.This is accompanied with the filling of the Asiatic main trough over Japan and the Northward shift of the Pacific subtropical high pressure belt from low latitudes to 30°-40°N.The mean date on which summer begins is nearly on 13th,July.It is also the end of Bai-Ü over middle Yangtze river.Thence the synoptic process in Bai-Ü period is the prevailing process before summer season.The precipitation in this period is closely related to the strength of the maximum wind belt i.e.socalled frontal zone.