during winter of which there are three to four occurrences every month in northern Vietnam. This leads to cold temperatures where temperatures drop by . Cold weather, occasionally extreme cold can persist for a long time, being characterized by a long stretch of cloudless or partly cloudy days in the first half of winter or a long stretch of cloudy and drizzly conditions in the latter half of winter. Cold weather occurs more frequently in the north than in the south due to cold fronts penetrate the north more frequently. 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. Spring and Fall are transitional seasons. The atmospheric circulation in these seasons represent a transition between winter–summer and summer–winter respectively. Mean annual temperatures in the country, based on meteorological data from weather stations range from in Hoang Lien Son. At the highest altitudes in the Hoang Lien Son range, mean annual temperatures is only . As temperatures vary by altitude, temperatures decrease by for every increase in altitude. The lowest mean annual temperatures are found in the mountainous areas where the altitude is higher and in northern areas due to their higher latitudes. Many mountainous areas in the north have experienced subzero conditions. In contrast, temperatures in the Spratly Islands never falls below . Because Vietnam is strongly influenced by the monsoon, the mean temperatures