Most of Thailand has a Tropical wet and dry or savanna climate (Aw) according to the Koppen climate classification, while the South and the eastern tip of the East have a tropical monsoon climate (Am); countrywide, temperatures normally range from an average annual high of 38 ^oC (100 ^oF) to a low of 19 ^oC (66 ^oF). During the dry season, the temperature rises dramatically in the second half of March, spiking to well over 40 ^oC in some areas by mid April when the Sun passes the Zenith. Southwest monsoons that arrive between May and July (except in the South) signal the advent of the rainy season, which lasts into October and the cloud covering reduces the temperature again but the high humidity is experienced as 'hot and sticky'. November and December mark the onset of the dry season and night temperatures on high ground can occasionally drop to a light frost. Temperatures begin to climb in January, and a hot sun parches the landscape. The dry season is shortest in the South because of the proximity of the sea to all parts of the Malay Peninsula. With only minor exceptions, every area of the country receives adequate rainfall, but the duration of the rainy season and the amount of rain vary substantially from region to region and with altitude. The Northeast experiences a long dry season although the dry 2007/2008 season lasted only from late November through mid March. Its red,(laterite) dense clayey soils retain water well, which limits their agricultural potential for many crops but is ideal for keeping the water in the paddy fields and local village reservoirs. The well drained, loose sandy alluvium of the Mekong flood plain is very fertile, the main crops being tomatoes on an industrial scale, tobacco, and pineapples.