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, eleven 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 six to eight typhoons or tropical cyclones per year.