In most populated areas in Thailand the mean monthly temperature ranges from the mid-70's (F.) for the coolest winter month (December or January) to the mid-80's for the warmest spring month (April or May). During the year the temperature seldom falls below 55° and rarely rises above 95°. The accompanying set of maps, illustrates temperature conditions during four months of the year--the cool season in January, the hot season in April, the middle of the rainy season in July, and the end of the rainy season in October. These may be compared with the set of rainfall maps for the same months.
Throughout the monsoon regions of Southeast Asia, the annual range of temperatures is less affected by the position of the sun than by the degree of cloudiness and the rainfall. In the continental portions of Thailand the warmest month is not July but April or May, preceding the rainy southwest monsoon. During the rainy season from June to October persistent cloudiness cuts off the sun's rays and lowers the average air temperature to between 80° and 85°. Radiation from the earth is also retarded by cloud cover, so that in the rainy season daily variations in the temperature are reduced. Daily temperatures in the Central Valley vary only from 4° to 6°. The high relative humidity of the monsoonal air masses, the almost daily showers that sweep across the land (particularly in the afternoons), and the humid hot nights combine to produce a greenhouse-like sultriness during the rainy season.
The rainy season in Northern Thailand mountains is not very different from the rainy season in the Central Valley, although cloudiness is probably somewhat heavier and continues longer, and average monthly temperatures are a few degrees lower. The southwest monsoon, as it sweeps up over the mountain heights, produces a continuous condensation of water vapor, a heavy cloud cover, and very small variations in the diurnal temperature range. The increased cloud cover obliterates the expected effects of increasing continentality in Northern Thailand, and, as a result, there is scant difference in rainy season temperatures between the two regions.
When the rainy season ends in October or November, cloudiness disappears, average temperatures begin to decline, and daily variations in temperature begin to increase. The winter season, with cooler temperatures and refreshing dry air, is the period Western visitors find most pleasant. Temperatures decline considerably more and show a greater range in the North than in the South. The nights in a northern city like Chiangmai may feel quite cool although temperatures reach the high 70's or low 80's during the daytime. Even in Bangkok a wool suit may be comfortable in January.
In February the "cool season" comes to an end, as air temperatures begin to rise with the increasing altitude of the sun. Though this rise is felt first in February, the real "hot season" commences in March; April and May bring the highest temperatures. During the hot days the sun's rays are unrelenting, and the hot nights give scant relief, although the Bangkok Plain may receive a sea breeze in the early evenings. Then the first rains of the southwest monsoon bring refreshing coolness until the sultry hothouse weather of the rainy season is well developed.
The period for traveling in the North is between December and the end of February. At that time of year the climate of the North Thailand mountains, even in the river plains, seems markedly fresher than in the south, and in March it does not seem so hot. By April and May, however, the daytime temperatures in Northern Thailand very closely approach those of the Bangkok Plain and daily variations are no longer much greater.
The temperature regime of Khorat is not very different from that of the Central Valley and the North. In the cool season, when the northeast monsoon blows unhindered across the Khorat plains, the over-all temperatures are probably lower than those of the Bangkok Plain. During the hot season they are somewhat higher.
In the Peninsula temperature conditions, owing to the short distances from the sea and the less distinct division of the year into dry and rainy seasons, are quite different from those of the rest of the Kingdom. Annual variation in temperature is very much lower. There is no cool season in the Peninsula as in the rest of the Kingdom. High temperatures prevail the whole year, and seasonal variation is largely a difference in relative humidity. The highest annual temperatures in the Peninsula occur just prior to the commencement of the monsoon, March and April on the West Coast and June and July on the East Coast.