Thailand, like all of southern Asia, is dominated by the monsoon (Arabic, mausim, meaning "season"). Monsoon winds are essentially seasonal winds blowing from one direction part of the year and from the opposite direction the remainder of the year. On this very simplified basis, four seasons may be recognized: (1) northeast monsoon from December through February--the "dry season"; (2) transitional hot weather and highly variable winds of March, April, and May; (3) southwest monsoon from May to October--the rainy season"; (4) retreating monsoon period of October and November.
Monsoon winds throughout Southeast Asia result from the interaction of two maritime air masses, the southern tropical and the northern tropical. The former, as the southeast trades, moves equatorward in the Southern Hemisphere, and the latter, as the northeast trades, moves equatorward in the Northern Hemisphere. The area of convergence between these air masses is known as the intertropical front. As this front follows the migration of the sun north and south of the equator during the Northern summer, the northeast trades, responding to Ferrell's Law, deflect to the right and become the southwest trades. Since both air masses are warm, slow-moving, and humid, this front cannot as a rule be sharply delimited. However, when the intertropical front is furthest from the equator, the contrast between the densities of the two trade winds is greatest and the front becomes somewhat more sharply defined. When this front extends over Thailand during the summer months, the warmer and more humid southwest winds are forced to ascend over the cooler and less humid northern air mass and storms of considerable violence may occur. During the winter period the front shifts far south of the equator and the northeast trades that cover all of continental Thailand are relatively calm and dry.
A monsoon is largely the result of the differential heating of land and water. During the transitional hot weather spring season, a large area of low pressure builds up over Southeast Asia. Into this trough move winds from the comparatively cooler high-pressure area over the adjoining sea. These winds have traveled great distances over water and although cooler than the land are still very warm. As a result, both their relative and absolute humidity are high and the season of the southwest monsoon is one of rain. In the winter monsoon, the land cools appreciably faster than the sea and a comparatively higher pressure area forms over Southeast Asia. This results in the reversal of winds, which now blow from the land mass toward the sea. Because of the protection afforded by the mountains of the Indo-Chinese states, the differential is not so great, so that the winter monsoon winds are much weaker. The mountains of Indo-China are also extremely effective in keeping most of Thailand rainless during the winter months. The northeast monsoon, which descends these slopes and moves through continental Thailand, is quite dry. Only the East Coast of the Peninsula receives a northeast monsoon that has just crossed a large water area and can furnish winter season precipitation.
It would be completely misleading to represent this atmospheric circulation too schematically. Sometimes the onset of the southwest monsoon is very definite; at other times, it is not easily discerned; and accurate prediction of its arrival is not yet possible in any area. Interruptions of the "normal" monsoonal air movements are very frequent. Some of the interruptions are traceable to relief and other local influences and some are determined by cyclonic atmospheric movements. Full understanding of the complex pattern awaits further studies of weather and climatic conditions in Southeast Asia.
During the winter and spring months increasing heat over the Southeast Asia land mass brings highly variable winds. Rising daytime temperatures over the Bangkok Plain after January produce sea breezes, which come at times from the south, at other times from the east. They may be considered as a deflected northeastern monsoon. This wind variability, especially from January to May, is felt over much of continental Thailand. When the wind changes are most intense, cool northern air and warm southern air meet energetically and can lead to the development of cyclonic rain. These are the "mango showers" which occur in Thailand irregularly in the drier seasons of the year, but most often in February or March and are of special significance for the growth of vegetation.
The southwest monsoon season is not one of completely unrelieved rainfall. There are frequent breaks in the monsoon, the number and length of which can be very important in deciding its agricultural effectiveness. Sometimes a break in the southwest monsoon may occur in August and several hot, humid, almost rainless weeks may pass before it resumes. Variations in the monsoon may occur nationwide but more commonly they affect only isolated parts of the country. The first type of variation is a late start; the second, a prolonged break; the third, an early termination; and the fourth, excessive concentration of rainfall in certain areas. The first three generally result in a deficiency of rainfall--often in drought and heavy crop failure. Extremely heavy falls in short periods may bring flash floods very damaging to agriculture. For example, abnormally high rainfall received in the Northern Thailand mountains is carried down the Čhaophraya tributaries where it often floods the Bangkok Plain excessively and drowns part of the rice crop.
Extra-tropical disturbances--typhoons--of considerable violence occasionally pass through the Gulf of Thailand from the South China Sea and strike the East Coast of the Peninsula, sometimes passing on into the Andaman Sea. Typhoons also come sometimes from the South China Sea, pass over the Annam Cordillera, and dissipate themselves in east and central Thailand. While the high winds of typhoons are seldom felt in Khorat, the area often does receive by-product rainfall from these disturbances. The peak season for South China Sea typhoons is from July to November.