Weather and Climate of Thailand
Thailand is located in the tropical area between latitudes 5 o 37 ′ N to 20 o 27 ′ N and longitudes 97 o 22 ′ E to 105 o 37 ′ E. The total area is 513,115 square kilometres or around 200,000 square miles.
The boundaries of Thailand with adjacent areas are :
North: Myanmar and Laos.
East: Laos, Cambodia and the Gulf of Thailand.
West: Myanmar and the Andaman Sea.
According to the climate pattern and meteorological conditions Thailand may be divided into 5 areas i.e. Northern, North-eastern, Central, Eastern and Southern Parts.
The topography of each area is quite different as follows:
This part is divided into 15 provinces i.e. Chiang Rai, Mae Hong Son, Chiang Mai, Phayao, Lamphun, Lampang, Phrae, Nan, Uttaradit, Phitsanulok, Sukhothai, Tak, Phichit, Kamphaeng Phet and Phetchabun. Most areas of the part are hilly and mountainous which is the source of several important rivers. These north-south oriented hill ridges are parallel from west to east and intersected by a number of major valleys, particularly those near Chiang Mai, Chiang Rai, Lampang and Nan provinces. The highest mountain, about 2,595 meters high above mean sea level, is Doi Inthanon in Chiang Mai. Along the eastern border with the North-eastern Part is mountainous called central highlands. The area in the southern portion between the western mountains and the central highlands is a central valley.
This region is naturally a high level plain called north-east plateau. North-west/south-east oriented Phu Phan ridge in the north-eastern portion separates this part into two basins. One is a large high level plain in the west. The another is smaller and slope towards the east. This part is divided into 19 provinces i.e. Nong Khai, Loei, Udon Thani, Nong Bua Lam Phu, Nakhon Phanom, Sakon Nakhon, Mukdahan, Khon Kaen, Kalasin, Maha Sarakham, Roi Et, Chaiyaphum, Yasothon, Amnat Charoen, Ubon Ratchathani, Sri Sa Ket, Nakhon Ratchasima, Buri Ram and Surin.
Central Part is divided into 18 provinces i.e. Nakhon Sawan, Uthai Thani, Chai Nat, Sing Buri, Lop Buri, Ang Thong, Sara Buri, Suphan Buri, Ayutthaya, Pathum Thani, Kanchanaburi, Ratchaburi, Nakhon Pathom, Nonthaburi, Bangkok Metropolis, Samut Prakan, Samut Sakhon and Samut Songkhram. This part is a large low level plain where the Ping, Wang, Yom and Nan Rivers originated in the Northern Part join together to be the Chao Phraya River at Nakhon Sawan province. However the western mountains in the Northern Part extend to this part along the western portion.
The south and south-west of the part is adjacent to the Gulf of Thailand. Farther in land, most areas are plains and valleys but there are some small hills in the northern, central and eastern portions. This part is divided into 8 provinces i.e. Nakhon Nayok, Prachin Buri, Sra Kaeo, Chachoeng Sao, Chon Buri, Rayong, Chanthaburi and Trat.
The topography of this part is the peninsula between the Andaman Sea which is on the western side of the part and the South China Sea which is on the eastern side. The long ridge of western mountains in the Northern and Central parts also extend to this part. Phuket ridge along the west coast and Nakhon Si Thammarat ridge in the central of lower portion forming the backbone of the Southern Part separate this part into two regions, Southern Thailand East Coast and Southern Thailand West Coast. Ten provinces from north to south which are Phetchaburi, Prachuap Khiri Khan, Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Pattani, Yala and Narathiwat belong to Southern Thailand East Coast while there are 6 provinces i.e. Ranong, Phang Nga, Krabi, Phuket, Trang and Satun in Southern Thailand West Coast.
General Climatic Conditions
The climate of Thailand is under the influence of monsoon winds of seasonal character i.e. south-west monsoon and north-east monsoon.
The south-west monsoon which starts in May brings a stream of warm moist air from the Indian Ocean towards Thailand causing abundant rain over the country, especially the windward side of the mountains.
Rainfall during this period is not only caused by the south-west monsoon but also by the Inter Tropical Convergence Zone (ITCZ) and tropical cyclones which produce a large amount of rainfall.
May is the period of first arrival of the ITCZ to the Southern Part. It moves northwards rapidly and lies across southern China around June to early July that is the reason of dry spell over upper Thailand.
The ITCZ then moves southerly direction to lie over the Northern and North-eastern Parts of Thailand in August and later over the Central and Southern Part in September and October, respectively.
The north-east monsoon which starts in October brings the cold and dry air from the anticyclone in China mainland over major parts of Thailand, especially the Northern and North-eastern Parts which is higher latitude areas.
In the Southern Part, this monsoon causes mild weather and abundant rain along the eastern coast of the part.
The onset of monsoons varies to some extent.
South-west monsoon usually starts in mid-May and ends in mid-October while north-east monsoon normally starts in mid-October and ends in mid-February.
From a meteorological point of view, the climate of Thailand may be divided into three seasons as follows:
Rainy or south-west monsoon season (mid-May to mid-October)
The south-west monsoon prevails over Thailand and abundant rain occurs over the country. The wettest period of the year is August to September. The exception is found in the Southern Thailand East Coast where abundant rain remains until the end of the year that is the beginning period of the north-east monsoon and November is the wettest month.
Winter or north-east monsoon season (mid-October to mid-February)
This is the mild period of the year with quite cold in December and January in upper Thailand but there is a great amount of rainfall in Southern Thailand East Coast, especially during October to November.
Summer or pre-monsoon season (mid-February to mid-May)
This is the transitional period from the north east to south-west monsoons. The weather becomes warmer, especially in upper Thailand. April is the hottest month.
Upper Thailand i.e. the Northern, North-eastern, Central and Eastern Parts usually experiences a long period of warm weather because of its inland nature and tropical latitude zone.
March to May, the hottest period of the year, maximum temperatures usually reach near 40.o C or more except along coastal areas where sea breezes will moderate afternoon temperatures.
The onset of rainy season also significantly reduces the temperatures from mid-May and they are usually lower than 40 o C. In winter the outbreaks of cold air from China occasionally reduce temperatures to fairly low values, especially in the Northern and North-eastern Parts where temperatures may decrease to near or below zero.
In the South, temperatures are generally mild throughout the year because of the maritime characteristic of this region. The high temperatures common to upper Thailand are seldom occur. The diurnal and seasonal variations of temperatures are significantly less than those in upper Thailand.
Upper Thailand usually experiences dry weather in winter because of the north-east monsoon which is a main factor that controls the climate of this region. Later period, summer, is characterized by gradually increasing rainfall with thunderstorms.
The onset of the south-west monsoon leads to intensive rainfall from mid-May until early October. Rainfall peak is in August or September which some areas are probably flooded. However, dry spells commonly occur for 1 to 2 weeks or more during June to early July due to the northward movement of the ITCZ to southern China.
Rainy season in the Southern Part is different from upper Thailand. Abundant rain occurs during both the south-west and north-east monsoon periods. During the south-west monsoon the Southern Thailand West Coast receives much rainfall and reaches its peak in September. On the contrary, much rainfall in the Southern Thailand East Coast which its peak is in November remains until January of the following year which is the beginning of the north-east monsoon.
According to a general annual rainfall pattern, most areas of the country receive 1,200 – 1,600 mm a year. Some areas on the windward side, particularly Trat province in the Eastern Part and Ranong province in the Southern Thailand West Coast have more than 4,000 mm a year. Annual rainfall less than 1,200 mm occurs in the leeward side areas which are clearly seen in the central valleys and the uppermost portion of the Southern Part.
Thailand is covered by warm and moist air in most periods of the year except the areas farther in land the relative humidity may significantly reduces in winter and summer.
For example, the extreme minimum relative humidity values shows only 9 % at Loei and Chiang Rai on 23 March 1983 and 23 April 1990, respectively. In the Southern Part which is maritime characteristic the humidity is relatively higher.
Cloud cover is normally less from November to March. Perfectly clear skies are generally found that is a reason why extreme temperatures usually occur. Most clouds in this period are high clouds but cumulus and cumulonimbus may be seen on some occasions. During the south-west monsoon, most clouds in the sky are convective clouds. Clear skies are seldom occur in this period except during June which have a few days.
For instance, the tropical storm “HARRIET” hit Nakhon Si Thammarat province in October 1962 and the typhoon “GAY” hit Chumphon province in November 1989 and the latest one was the typhoon “LINDA” which hit Prachuap Khiri Khan province in November 1997 as it was tropical storm.
By considering the annual mean, tropical cyclones usually move across Thailand about 3 – 4 times a year. During January to March, Thailand has never received the effect. According to the historical data, it can be seen that April is the first month which tropical cyclone move across Thailand.
The relatively higher frequencies are found from May, particularly September and October. They usually pass through the Northern and North-eastern Parts in early south-west monsoon season and will move across the southern Thailand from October to December.