Like most of Thailand, Bangkok has a tropical savanna climate (Aw) under the Köppen climate classification and is under the influence of the South Asian monsoon system. It experiences three seasons: hot, rainy, and cool, although temperatures are fairly hot year-round, ranging from an average low of  in December to an average high of  in April. The rainy season begins with the arrival of the southwest monsoon around mid-May. September is the wettest month, with an average rainfall of . The rainy season lasts until October, when the dry and cool northeast monsoon takes over until February. The hot season is generally dry, but also sees occasional summer storms. The surface magnitude of Bangkok's urban heat island has been measured at  during the day and  at night. The highest recorded temperature of Bangkok metropolis was  in March 2013, and the lowest recorded temperature was  in January 1955.