In summer, the general wind pattern are southwesterly winds in the southern parts of Vietnam and southeasterly winds in northern Vietnam.: The predominantly air blocks in Vietnam are the equatorial and tropical blocks that originate from high pressure systems in the Southern Hemisphere, and a maritime tropical block originating from the subtropical high pressure system in the Pacific Ocean (Pacific subtropical high pressure).: In addition, during summer, Vietnam is influenced by tropical air from the Bay of Bengal which occurs when a continental low pressure originating from South Asia (South Asian continental low) moves eastwards towards Vietnam, covering almost all of Vietnam and southern China; this causes hot, dry weather in the North Central Coast as westerly winds descend and warm adiabatically on the eastern slopes of the Annamite Range (Truong Son range).: On average, 11 storms and tropical low pressures develop in the South China sea during summer of which half are tropical cyclones that originate from the western Pacific.: These storms and cyclones then move westwards towards Vietnam.: On average, Vietnam is affected by 6-8 typhoons or tropical cyclones per year.: