On the basis of rainfall and temperature conditions and the patterns of vegetation, most of continental Thailand and parts of the Peninsula can be classified as Tropical Savanna (Köppen "Aw"). In the far northeastern part of North Thailand higher elevations produce sufficiently cool temperatures to place most of the area in the humid subtropical zone ("Cw"); and in sections of the Southeast Coast and the Western Mountains heavier rainfall produces a Tropical Monsoon climate ("Am"). Areas in the Peninsula south of Pračhuapkhirikhan čhangwat, where rainfall is very heavy, also fit the Tropical Monsoon type. This includes all of the West Coast of the lower Peninsula and all except a few lowland coastal sections of the East Coast. Although vegetation in part of the rainy West Coast as well as the Southeast Coast of the mainland approaches or reaches the true rainforest type, these areas fail to get the Köppen designation of Tropical Rainforest ("Af") climate, because there are two or three months of a dry winter season when the rainfall is under 2.4 inches.
For descriptive purposes the major physiographic regions can also be taken as climatic regions, as they tend to coincide approximately.
The climate of the Central Valley is affected by lack of relief and nearness to the sea. The average annual rainfall is 52.9 inches, and only along the margins of the valley are there differences in the general rainfall pattern of the area. Everywhere major precipitation is received in the southwest monsoon from May through October; the remainder of the year is comparatively dry, although a month without any precipitation at all is rare. During the summer season, the Central Valley receives an average fall of 39-44 inches. Since rice under local Thai conditions requires 71 inches to mature properly, the area must depend heavily upon irrigation or natural flooding.
Temperature patterns are also very similar throughout the region. During the transitional hot weather of March, April, and May, the thermometer may rise above 100° F. In winter it may reach a low in the 40's.