Southern Thailand Geography & Climate

Southern Thailand occupies the Malay Peninsula from the Isthmus of Kra down to the Malaysian border, a long, narrow strip with sea on both sides and a spine of hills down the middle.

Two coasts

The Andaman coast in the west is the dramatic one: limestone karst eroded into sheer towers and islands, deep clear water, and cliffs undercut at sea level by millennia of erosion. The Gulf coast in the east is gentler, with long sandy beaches, shallower water and granite rather than limestone islands.

The Isthmus of Kra

At its narrowest the peninsula is only about 44 kilometres across, and schemes to cut a canal there have been proposed since the seventeenth century without ever being built.

Karst

The limestone is the region’s signature. Rain and sea have hollowed it into caves, sinkholes and hongs — collapsed chambers open to the sky, reachable only by paddling through a tunnel at low tide. Phang Nga Bay is the classic expression of it.

The interior

The Nakhon Si Thammarat and Phuket ranges run down the peninsula, holding the last substantial lowland rainforest in the country. Khao Sok preserves a forest of great age, and Cheow Lan lake, created by a dam in 1982, drowned its valleys and left limestone peaks standing in open water.

The 2004 tsunami

The Indian Ocean tsunami struck the Andaman coast on 26 December 2004, killing more than five thousand people in Thailand, with Khao Lak worst hit. Warning towers and evacuation routes date from it.

Southern Thailand has a tropical monsoon climate with no cool season at all — the temperature sits between about 25 and 33°C every month of the year — and what changes is the rain, which arrives on the two coasts at different times.

The Andaman side

Exposed to the south-west monsoon from roughly May to October, when rain is heavy, seas are rough and some island services stop. Its dry season runs November to April, which is the high season for Phuket, Krabi and the Similans.

The Gulf side

Sheltered from the south-west monsoon by the peninsula’s spine but exposed to the north-east monsoon, which brings its heaviest rain from October to December. Samui, Phangan and Ko Tao are therefore at their best from February to September — almost the inverse of the west coast.

Why this matters

A trip planned for the wrong coast in the wrong month is the commonest mistake made in the Thai south. November is superb on the Andaman and frequently washed out on the Gulf; July is the reverse.

Rainfall and humidity

Totals are high — commonly 2,000 to 2,500 millimetres a year, and considerably more on the Andaman slopes — and humidity stays around 75 to 85 per cent throughout. Rain usually comes in intense bursts rather than all day.

The wet season is not a write-off

Low-season rain here rarely falls all day. A typical monsoon pattern is a clear morning, a violent downpour in the afternoon and a fine evening, and the islands are greener and half the price. What the monsoon genuinely costs you is sea state: rough crossings, cancelled ferries, closed dive sites and poor visibility underwater. If the trip is about boats and reefs, the season matters enormously; if it is about a beach and a book, much less.

Storms and the sea

Tropical storms occasionally cross the Gulf, and rough seas suspend ferries at short notice in either monsoon.