Thailand is a tropical country. As in the whole of South-East Asia, the climate is affected by the monsoon. The monsoons are seasonal winds which change direction during the year as a result of differences in temperature and pressure between land and sea. In the spring, a large zone of low pressure develops over South-East Asia and the winds blow off the sea, which is relatively cooler, on to the land, their relative and absolute humidity being very high. As a result this south-western monsoon produces the rainy season. In winter, on the other hand, the land cools more rapidly than the sea, and a zone of high pressure builds up over South-East Asia. This winter monsoon leads to a flow of air from the land to the sea, from north-east to south-west; and this produces Thailand’s dry season, except on the south-eastern coast, which the winds reach only after they have passed over the Gulf and stored up a certain amount of humidity. On the basis of these changes in wind direction it is possible to identify three seasons in Thailand:
(1) a dry and cool season (the north-east monsoon) from November to February;
(2) a transitional season with higher temperatures and variable winds from March to June; and
(3) a hot and humid season from July to October (the summer monsoon);
There are of course considerable local variations within this pattern. For Thailand, dependent on water for its rice-growing, the rainfall (or lack of rainfall) is of more importance than the temperature. The range of temperature variation is in any event fairly limited (under 10° at Bangkok) and is fairly predictable in the different parts of the country. This cannot be said of the country’s rainfall, which shows considerable variations from year to year and from area to area.
In all parts of the country the rainy season coincides with the period when the south-western monsoon is blowing (May to October); but the quantity of rainfall in different areas shows remarkable variations from the average, mainly determined by the local pattern of relief. On the western slopes of the Tenasserim coast, on the Burmese frontier, there are places with a rainfall of 4 or 5 metres a year. The effect of the orographic factor can be seen from the fact that on the eastern slopes, round Kanchanaburi, the annual rainfall is barely 200 mm. Moreover since the winds discharge the rain so generously on the coastal areas the rainfall declines steadily towards the interior. This pattern is of great importance for agriculture and vegetation: dense forests in the areas exposed to the summer monsoon, open dry forests in the interior, forests of transitional type between the two.
The driest parts of Thailand are in the western part of the Khorat plateau, particularly in the lee of the mountain ranges between the Khorat area and the Menam plain. Another dry area is the region lying at the foot of the western hills, extending from the district of Prachuap Khiri Khan to the beginning of the Malay peninsula by way of Ratchaburi, Phetchaburi, Kanchanaburi and the province of Tak. The wettest area is the west coast of the Malay peninsula, where the monsoon from the Andaman Sea strikes the high coastal hills. At Takuapa the rainfall is more than 4 metres a year.
Temperatures in the most populous parts of the country range between 25°C (December, January) and 30°C in the hottest months. As noted above, the limited range of variation is characteristic of Thailand. At Bangkok the temperature never falls below 13°C (morning temperature from November to January about 18°C and never goes above 35°C in the shade. During the wet season the fact that the sun is at or near the zenith in the Tropic of Cancer is of less importance in determining the temperature than the amount of cloud cover. As a result the variation of temperature during the day is no more than 3° to 5°C. Since the humidity of the air then approaches 100 – rising during the day, with regular daily showers between 3 and 5 in the afternoon – the climate between May and October is very hot and humid.
About October-November the cloud systems begin to break up and the daily temperature range increases; at this time it is much cooler in the north of the country. Then the dry winter begins, a time of year eminently suitable for tourism, particularly for European and American visitors.
In February the temperature begins to rise gradually, and in March the hot season begins, April and May being being extremely hot. The sun beats down implacably, and even the nights provide little relief. At most the plain round Bangkok gains a little coolness in the evening from the breezes blowing off the sea. In the Nakhon Ratchasima region, with its more continental climate, the summer is hotter and the winter cooler than in the Bangkok basin.
The climate of the Malay peninsula is no less unique to that region than its geology and morphology. Lying on the sea, it has much less marked differences between the seasons than the rest of Thailand; the annual temperature range is low and the climate is sub-equatorial. From time to time typhoons engendered by areas of low pressure in the South China Sea cut diagonally across the Gulf of Thailand and hit the cast coast of the peninsula, sometimes continuing into the Andaman Sea. Cyclones can also move north and cross the mountains of Annam, into eastern or central Thailand, mostly between July and November.
Thus on the basis of temperature and rainfall the country can be divided into three climatic zones. Most of the interior of Thailand and part of the Malay peninsula have a dry tropical climate; the higher altitudes of the north-east produce a humid subtropical climate; and the rest of the country has a tropical monsoon climate.