Rainfall--its amount, distribution, timing, and intensity--is far more important to man in Thailand than is temperature. Temperature conditions in this tropical land change relatively little during the year, and the temperature pattern of any one area is rather closely predictable. Precipitation, on the other hand, is highly unpredictable, and radical departures from the average are common, both from place to place and from year to year in a given location.
The accompanying set of maps illustrate rainfall conditions during four months of the year--the northeast monsoon in January, the transitional season of April, the middle of the southwest monsoon in July, and the end of the southwest monsoon in October.
Most of Thailand receives the bulk of its rain during the period of the southwest monsoon, May through October. The single exception is the Peninsula East Coast. The quantity of rainfall is largely a function of local relief.
Orographic influences are everywhere important. Along the rainy western slopes of the Central Cordillera on the Tenasserim coast of Burma, most stations record over 150 inches of precipitation annually. So effectively have these mountains removed the moisture from the air that on the eastern side of the Cordillera at Kančhanaburi the average annual rainfall is about 43 inches. On the eastern side of the Bangkok Plain where winds are forced to ascend the Khorat escarpment that rises abruptly from sea level, they deposit over 80 inches of rain at Nakhonnayok. The higher mountain slopes around Khorat have similar rainfall increases, reflected in the presence of dense forest vegetation. Near the coast of Southeast Thailand are groups of mountains and ranges whose relief and proximity to the ocean cause heavy precipitation at their bases. In Čhanthaburi, for example, at the foot of the Banthat range, the average annual rainfall is 98.2 inches, while not far westward along the coast at Chonburi, a region without strong relief, the rainfall is only 47 inches.
The quantity of rainfall decreases with increased distance from the sea-a phenomenon reflected clearly in the vegetative cover: evergreen forests in the extreme oceanic type; true monsoon forest farther from the coast; monsoon dry forests in the more continental portions of the country. The average annual rainfall in the Central Valley gradually decreases from south to north. Bangkok has 59 inches, Lopburi has 54 inches, and Nakhonsawan in the Upper Valley averages 42 inches. Towards the northern end of the Upper Valley, however, rainfall increases as a result of the orographic effect of the northern mountains, and Phitsanulok has 57 inches each year. Within these mountains the precipitation diminishes inland and away from the plains. Thus, 165 inches often falls each year in the heart of the Central Cordillera along the West Coast of the Peninsula, but in Northern Thailand mountains, far from the ocean, average rainfall is less than 80 inches annually. Inhabited valley areas generally have much less: Chiangmai receives 40 inches and Nan about 50 inches.
The driest regions in Thailand are found in western Khorat, a section in the rain shadow of the escarpment and hills between Khorat and the Central Valley, and along the base of the Western Mountains from Pračhuapkhirikhan čhangwat in the upper Peninsula through Phetchaburi, Ratchaburi, Kačhanaburi, and Tak čhangwat, an area in the rain shadow of the high Central Cordillera.
Along the Peninsula in Chumphon čhangwat and southward, rains occur at all seasons. Because of the narrowness of the Peninsula, the several mountain ranges receive rain from the southwest as well as the northeast monsoon. As there is always an increase in the total rainfall when the monsoon winds blow in from the sea, the largest quantities fall along the West Coast from May to October during the southwest monsoon and along the East Coast from October to January during the northeast monsoon. The greatest quantities of rain fall along the West Coast of the Peninsula where the monsoon sweeps in from the Andaman Sea and strikes high mountains closely paralleling the coast. North of Phuket and behind Takuapa and Ranong are hill slopes which rise rapidly to 3,000 and 3,500 feet. Here, air masses saturated with water vapor cause extremely heavy rains. Takuapa receives 166 inches a year, and one year received 260.1 inches.
Variability of rainfall is closely connected with the annual average amount. Generally, the lower the average amount received, the greater will be the variability of its receipt. Thus, markedly higher degrees of variability have been measured at stations in Khorat. One of the reasons for this may be the heavy contributions from convectional thunderstorms. Convectional thunderstorms are also common in the Central Valley, and there is considerable variability there as well. This can be appreciated when the Bangkok Plain is seen from the air in April or May. Numerous spots of green grass or plowed areas are evident before the general rains have commenced. In the Peninsula and in the north, as might be expected, precipitation varies less from place to place or from year to year.
Another variable element in the pattern of precipitation is the duration of the rainy season. During the period 1906 to 1925 the average date of the first rain of the season was April 25 while the average date of its ending was November 12, a total avenge duration of 201 days. The longest rainy season was 236 days and the shortest 174 days during this nineteen-year period. (These figures are deceptive, however, for rains received in April may be followed by a long dry period before the monsoon commences in earnest around the first of June.) In any case, the duration of the rainy season is as important as the amount, distribution, and intensity of the rainfall for the agriculture of Thailand.