Northern Thailand
Northern Thailand Geography & Climate
Geography
Northern Thailand is the country’s upland region, where a series of parallel mountain ranges run roughly north to south as the southern extremity of the system that begins in the Himalayas.
The ranges and valleys
The Thanon Thong Chai, Daen Lao and Phi Pan Nam ranges divide the region into long north-south valleys, and almost every town of consequence sits in one — Chiang Mai in the Ping valley, Chiang Rai on the Kok, Nan on the Nan. Doi Inthanon, at 2,565 metres, is the highest point in Thailand, cool enough at the summit for moss forest and rhododendron rather than anything tropical.
The rivers
The Ping, Wang, Yom and Nan all rise here and converge to the south to form the Chao Phraya, the river that made central Thailand. The Mekong forms the northern border with Laos, and at the Golden Triangle it meets the Ruak where Thailand, Laos and Myanmar touch.
Limestone
Karst outcrops rise abruptly from the valley floors, riddled with caves — Tham Lod near Soppong, and the Tham Luang cave system in Chiang Rai province, where a football team was trapped for eighteen days in 2018.
Forests
The north holds most of Thailand’s remaining forest cover, mixed deciduous and dipterocarp at lower levels grading into evergreen and pine higher up, with teak historically the great commercial tree.
Why the valleys matter
Almost nothing about settlement here is arbitrary. The ranges run north to south, so the valleys between them are the only easy routes, and every historic kingdom, trade road and modern highway follows one. It is also why the region divides so cleanly into provinces that each occupy a basin, and why two towns thirty kilometres apart as the crow flies can be half a day apart by road.
Borders
Myanmar lies to the west and north, Laos to the north-east, and both borders run through mountains that have never been easy to police.
Climate Patterns
Northern Thailand has a tropical savanna climate modified considerably by altitude, and it is the one part of the country with a genuinely cool season.
The cool season
November to February is dry and clear, with Chiang Mai around 28 to 30°C by day and dropping to 13 to 16°C overnight. In the mountains it is colder — Doi Inthanon’s summit regularly falls to freezing and has recorded frost, which Thais travel to see. A jacket is genuinely needed in the evenings, which surprises people packing for the tropics.
The hot season
March to May brings 35 to 40°C with low humidity, and April is the hottest month of the year.
The burning season
This is the north’s defining seasonal problem. From roughly February to April, agricultural burning across the region and across the borders fills the valleys with smoke, trapped by the surrounding mountains and by temperature inversions. Chiang Mai’s air quality regularly ranks among the worst of any city in the world during these weeks, with readings well into hazardous ranges. Anyone with a respiratory condition should avoid the region entirely then.
The wet season
June to October brings the southwest monsoon: heavy but usually short afternoon downpours, around 1,000 to 1,200 millimetres a year, with September the wettest month. The mountains are green, the waterfalls full, and the air is clean.
What this means for timing
The north rewards precision about dates more than most places. November to early February is close to ideal and is the only window that combines clear air, comfortable temperatures and dry roads for the mountain loops. The four weeks either side of that are fine. March and April are the ones to avoid outright, and not because of the heat — the smoke is the reason, and it is the single most common cause of a disappointing trip here.
Altitude
Expect roughly six degrees cooler for every thousand metres climbed.