Two characteristic trends in temperature variation fit into the two broad areas of Thailand. The upper mainland, i.e. the Northern, North eastern, Central and Eastern Parts usually experience a long period of warm weather because of its inland nature and tropical latitude zone. From March to May, the hottest period of the year, maximum temperatures usually reach near 40oC or more except along coastal areas where sea breezes will moderate afternoon temperatures. The onset of the rainy season also significantly reduces the temperatures from mid-May and they are usually lower than 40oC. In winter the outbreaks of cold air from China occasionally reduce temperatures to fairly low values, especially in the Northern and North eastern parts where temperatures may decrease to near or below zero at high altitudes at night time. 

          In the Southern Part temperatures are generally mild throughout the year because of the maritime characteristic of this region. The high temperatures common to upper Thailand seldom occur. The diurnal and seasonal variations of temperatures are significantly less than those in upper Thailand.