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 in Vietnam are lower than other countries located at the same latitude in Asia. The highest temperatures normally occurs in March–May in the south and May–July in the north. Temperatures in summer are relatively equal among the northern and southern parts of the country with differences being mostly due to altitude (the decrease in temperature is predominantly due to altitude).