Central Thailand Geography & Climate

Central Thailand is the alluvial basin of the Chao Phraya, a flat expanse of some of the most productive farmland in Southeast Asia, bounded by hills to the west and the Khorat plateau to the east and opening south to the Gulf of Thailand.

The river system

The Ping, Wang, Yom and Nan rise in the northern mountains and converge near Nakhon Sawan to form the Chao Phraya, which then runs some 370 kilometres south through Ayutthaya and Bangkok to the Gulf. The plain it has built is almost entirely flat — Bangkok sits around 1.5 metres above sea level — and is criss-crossed by an irrigation and canal network developed over centuries.

The delta

South of Bangkok the land becomes mangrove, salt pan and shrimp farm before reaching the Gulf, and the coastline is still advancing seaward in places and eroding badly in others.

The western hills

Toward Myanmar the ground rises into the Tenasserim range, limestone and forested, cut by the Khwae Noi and Khwae Yai rivers — this is the Kanchanaburi country, with caves, waterfalls and the largest protected forest complex in the region at Thungyai and Huai Kha Khaeng, UNESCO-listed and holding tigers.

Sinking and flooding

Bangkok is subsiding, historically through groundwater extraction, at a rate that combined with sea-level rise makes serious flooding a structural problem. The floods of 2011 inundated large parts of the plain for months.

Limestone outliers

Isolated karst hills rise abruptly from the flat land around Lopburi and Saraburi, honeycombed with cave temples.

Central Thailand has a tropical savanna climate with three distinct seasons, and unlike the north it has no altitude to soften any of them.

The cool season

November to February is the relief: highs around 31 to 33°C, overnight lows near 21°C, low humidity and very little rain. Bangkok is genuinely pleasant in December and January, and this is when everyone comes.

The hot season

March to May is severe. Temperatures reach 35 to 40°C with humidity that keeps nights above 27°C, and the apparent temperature in the city routinely exceeds 45. April is the hottest month in the Thai year. Sightseeing has to be done before ten and after four.

The wet season

June to October brings the southwest monsoon, with around 1,400 millimetres of rain falling mostly in intense late-afternoon storms. September and October are wettest, and this is when Bangkok’s drainage is tested — street flooding after an hour of rain is routine, and serious river flooding occurs in bad years.

Air quality

Bangkok’s fine-particle pollution is worst from roughly December to March, when cool stable air traps traffic and agricultural smoke. It is less extreme than the northern burning season but affects the whole cool-season window.

The 2011 floods

An exceptional monsoon put much of the plain under water for months, reaching the northern edges of Bangkok, closing industrial estates that supplied a large share of the world’s hard-disk drives, and displacing millions. It remains the reference point for what the wet season can do here, and the flood defences built since are a visible feature of the landscape north of the city.

Practical effect

Everything indoors is air-conditioned hard, and the swing between a 35°C street and an 18°C shopping centre is the thing most likely to give you a cold.