Best time to visit: Phuket, Khao Lak or Koh Samui — the two coasts have opposite monsoons

Southern Thailand does not have one rainy season but two, and they are almost opposites. Phuket and Khao Lak on the Andaman coast are dry from November to April; their monsoon falls between May and October. Koh Samui in the Gulf of Thailand works the other way round: its wettest stretch is October to December, with November the wettest month of all — exactly when the Andaman side is entering its best season. That is why a general answer to "best time to visit Thailand" is right for one island and wrong for the other.

The short answer

• ·November to April → Phuket and Khao Lak. Calm sea, clear water, national parks open.
• ·February to April → both coasts. The safest window if you want Phuket and Samui.
• ·June to September → Koh Samui; the Andaman side is in monsoon.
• ·October to December → avoid Samui if you need beach weather; November is its wettest month.

Why

From May to October the southwest monsoon drives moisture against the west coast — Phuket, Khao Lak, Krabi. Around November the wind turns: the northeast monsoon brings rain to the eastern side into January, which means the Gulf of Thailand, where Koh Samui, Koh Phangan and Koh Tao lie. The two coasts are only about 200 kilometres apart as the crow flies, but a mountain range and two wind systems separate them.

Month by month

• ·November: Phuket and Khao Lak open their season, the sea settles, the national parks have been open since mid-October. Koh Samui has its wettest month — rain arrives in heavy bursts rather than all day, but boat trips are cancelled more often.
• ·December to January: high season on the Andaman side, pleasant temperatures, busy over Christmas and New Year. On Samui the rain eases through January.
• ·February to April: the best time for both coasts. Clear water, calm sea; April gets hot (above 35 °C is normal).
• ·May to June: the monsoon starts on the Andaman side and prices fall; the Similan and Surin Islands close on 15 May. Samui is dry and pleasant.
• ·July to September: the classic window for Samui and Koh Phangan. Phuket and Khao Lak see rain, usually in one- or two-hour bursts — land tours carry on, boat trips not always.
• ·October: the turn. The Andaman side dries out and from 15 October the Similan and Surin national parks reopen. On Samui the wet phase begins.

The national parks keep their own season

This is what climate tables do not show: the Similan and Surin Islands are national parks, open only from 15 October to 15 May. The same goes for Koh Rok, Koh Kradan and the Krabi islands. Come to Khao Lak in July and you cannot visit the Similans, whatever the weather does. Phi Phi and Phang Nga Bay, by contrast, run all year.

In the Gulf that restriction does not exist: Angthong Marine Park and Koh Tao are reachable year round; only the swell decides.

Phuket or Koh Samui — by month

• ·in October: Phuket, narrowly. Samui is entering its wet phase.
• ·in November: Phuket or Khao Lak, clearly.
• ·in December and January: the Andaman side; Samui improves but is not reliable.
• ·in February, March, April: either. For snorkelling, the Andaman side (Similan, Surin).
• ·in May and June: Samui.
• ·in July and August: Samui — the European summer holidays land well here.
• ·in September: Samui. September is the wettest month on the Andaman coast.

What rain actually means

Tropical rain is rarely a grey day: it usually falls in heavy bursts of one or two hours, often in the afternoon or at night, and then the sun returns. For the beach that means waiting; for boat trips the problem is the swell, not the rain. Whether a tour runs is decided by the operator in the morning, on the day's weather and safety conditions. More in the rainy season in southern Thailand and in Thailand from A to Z.