The Spring Thunderstorms of the Szechwan Basin

The thunderstorms of Szechwan mainly occur at the evening in spring season. Most of them originate in SW portion of the a asin then move NE-ward, and finally decay in the eastern hills, heavy showers, gale winds and even hailstones are usually accom-panied with them. Before the coming of the thunderstorm, the weather is clear, sultry and moist. The air pressure falls slowly, light S-ly winds prevail. As soon as it approaches, the pressure tends to rise but unsteadyly, the wind suddenly turns to N or NW and much stronger than before. Both the temperature and the abcolute humidity fall rapidly, After its passage, the wind becomes feeble and variable, the temperature still stands high,but the absolute humidity continuously decreases. The middle and the lower clouds are mostly lenticular in form. The air mass in lower atmosphere is tropical in origin, The oscillation of the pressure reveals the existence of diffrent air mass in the upper layer. According to the pilot balloon observations, the air mass existing in the upper layer mostly comes from the NW plateaux. The fall of humidity and the presence of the lenticular clouds show that the air has subsided during the passage of the thunderstorm. From above, it is obvious that, at first, the tropical air mass moves N-ward in the lower atmosphere, meanwhile, the dry and cold air mass of the plateaux pushes toword SE in the upper air. such an arrangement is naturally unstable. In the afternoon,the ower layar gets warmer due to the contact with the hot earth surface, but the dry upper layer is cooled by the strong night radiation as the evening approaches. So that the instablity of the atmosphere is greatly increased in this time. Eventually the air column must be overturned and the thunderstorm forms. The direction of the storm follows that of the tropical air current.