The boundaries of the Central Valley climatic area to the west and to the east are not easy to delineate. Both the steep mountains in the west and the ascent to Khorat in the east have a marked influence upon the neighboring portions of the Central Valley. The Western Mountains cast a rain shadow along their side of the plains, and a marked deficit of rainfall characterizes a narrow belt extending from the Bight of Bangkok through Rahaeng on the Ping River. The average annual precipitation at Ratchaburi is 43 inches, at Kančhanaburi, 43 inches, and at Rahaeng, 37 inches, definitely below the average of the Central Valley. As far north as Rahaeng it is possible to see the effects of this rain shadow on the vegetation; dry bamboo forests are interspersed with thornbrush.
In contrast to the lower rainfall along the western margins of the Central Valley, the eastern margins experience a higher rainfall. In summer the southwest monsoon, blowing from the Gulf of Thailand inland over the Bangkok Plain, strikes the steep ranges along the southern and western corners of Khorat. At the southwest corner of Khorat where these two ranges meet is the highest point of the region, Khao Laem, with an altitude of 4,500 feet. Rainfall is almost as heavy at this steep point as along the coastal ranges of peninsular Thailand. Heavy rainfall is reflected in luxuriant tall forests. At stations to the west and south of these slopes annual recorded rainfall is 67 inches in Saraburi; 83 inches in Nakhonnayok; and 75 inches in Pračhinburi. Estimates of as much as 100 inches of rainfall annually on the upper southand west-facing mountain slopes seem justified.