At this time if a trough in the south branch of the westerlies moves eastward over this cold cushion, then there offen appears a trough and frontogenesis in the region surface high.

• 在高空平直的西风环流下,沿南支西风急流带,有低压槽在冷空气垫之上东移,地面高气压区域中常有低压槽和锋生出现。