Upper Thailand usually experiences dry weather in winter because of the northeast monsoon which is a main factor that controls the climate of this region. The summer is characterized by gradually increasing rainfall with thunderstorms. The onset of the southwest monsoon leads to intensive rainfall from mid-May until early October. Rainfall peaks in August or September when some areas are often flooded. However, dry spells commonly occur for 1 to 2 weeks or more during June to early July due to the northward movement of the ITCZ to southern China. 

          The Rainy season in the Southern Part is different from upper Thailand. Abundant rain occurs during both the southwest and northeast monsoon periods. During the southwest monsoon the Southern Thailand West Coast receives much rainfall and reaches its peak in September. On the contrary, most rainfall in the Southern Thailand East Coast reaches itspeak in November remaining until January of the following year, which is the beginning of the northeast monsoon.

          According to a general annual rainfall pattern, most areas of the country receive 1,200-1,600 mm a year. Some areas on the windward side, particularly Trat Province in the Eastern Part, and Ranong Province on the Southern Thailand West Coast have more than 4,000 mm a year. Annual rainfall with less than 1,200 mm occurs on the leeward side areas which are clearly seen in the central valleys and the uppermost portion of the Southern Part.