Isaan - Northeast Thailand Geography & Climate

Isaan occupies the Khorat Plateau, a raised sandstone tableland covering about 170,000 square kilometres — roughly a third of Thailand — and tilted gently from its western and southern rims down toward the Mekong.

Edges and escarpments

The plateau is separated from the central plain by the Phetchabun and Dong Phaya Yen ranges in the west and from Cambodia by the Dangrek escarpment in the south. That southern rim is a cliff line rather than a range, and the Khmer temples of the region sit along it or just behind it, controlling the passes.

Rivers

The Mun and its tributary the Chi are the main rivers, draining the plateau eastward into the Mekong, which forms much of the region’s northern and eastern border with Laos. At Khong Chiam the Mun enters the Mekong and the different sediment loads keep the waters visibly separate for some distance.

Poor ground

The plateau’s soils are sandy, thin and in places saline — rock salt lies beneath much of the surface and is extracted commercially. Combined with unreliable rainfall, this is why Isaan has always been the poorest farming region in the country despite its size.

Table mountains

Isolated sandstone massifs rise from the plateau at Phu Kradueng and Phu Hin Rong Kla, flat-topped, with pine forest and grassland on top and cliffs at the edges.

Salt beneath the plateau

A thick layer of rock salt underlies much of the Khorat basin, laid down when an inland sea evaporated, and it comes to the surface in places as saline patches where almost nothing grows. Villages have extracted it for centuries by flooding the ground and boiling the brine, and it remains a small local industry — but the same salt is one reason the plateau’s farmland is so much poorer than the central plain’s.

Dinosaurs

The sandstone has yielded some of Southeast Asia’s most important dinosaur fossils, with museums at Kalasin and Khon Kaen built on the excavation sites.

Isaan has a tropical savanna climate, and because it is inland, elevated and far from any sea, it experiences the widest temperature range in Thailand.

The cool season

November to February is dry and, by Thai standards, genuinely cold. Daytime highs of 29 to 32°C drop to overnight lows of 12 to 16°C across the plateau, and the table mountains at Phu Kradueng and Phu Ruea record the lowest temperatures in the country, occasionally near freezing with frost. Isaan people own coats, which surprises visitors.

The hot season

March to May is the hottest in Thailand. April routinely exceeds 40°C, the record temperatures for the country have been set on this plateau, and the humidity is lower than the coast but the heat is fiercer. The ground cracks, reservoirs drop and the landscape turns brown.

The monsoon

June to October brings the rains, but Isaan sits in a partial rain shadow and receives less than the rest of the country — commonly 1,000 to 1,400 millimetres, and it arrives unreliably. A late or short monsoon means a failed rice crop, which is why the Rocket Festival exists.

Drought and flood

The region suffers both, sometimes in the same year: drought in the dry season as reservoirs empty, then flash flooding when heavy rain runs off hard-baked ground. The Mekong’s levels have also become less predictable since upstream dams were built.

Practical effect

Pack a warm layer for a December visit and plan April sightseeing for before nine in the morning.