Destination timing · C11

Thailand has two rain calendars: Phuket and Krabi are not Koh Samui

“Thailand in July” is not yet a useful climate question. Phuket and Krabi face the Andaman Sea; Koh Samui sits across the peninsula in the Gulf of Thailand. Their temperatures…

Kalksteinfelsen und Strand bei Phuket — Photo: PavF · CC BY-SA 3.0

The 20-second decision

Travel window	Phuket and Krabi	Koh Samui	What it means
January to March	strongest shared window	strong as well	choose by trip style, not rain avoidance
July and August	16–17 rainy days, scores 60–62	8–9 rainy days, scores 73–74	Koh Samui has the clearer climate advantage
September and October	wettest part of the west-coast calendar	decline arrives later	warm sea does not make the coasts equivalent
November and December	marked recovery in the west	November is Samui's wettest month	Phuket and Krabi take over late in the year

These are historical monthly averages for 1991–2025, not a promise for a particular week.

The sea creates a false tie

Water temperatures are warm throughout the comparison. In July, the stored sea series is about 29.4 °C for Phuket, 29.8 °C for Krabi and 29.7 °C for Koh Samui. Looking at that column alone produces three near-identical beach choices.

Rain frequency breaks the tie. Phuket and Krabi each average about 16 rainy days in July, compared with eight on Koh Samui. Monthly precipitation is approximately 267, 249 and 172 millimetres respectively. A rainy day does not mean uninterrupted rain, but a holiday built around boats, beaches and a tight schedule has less margin when wet days are twice as frequent.

That is why Koh Samui scores 74 in July while Phuket and Krabi score 62 and 60. The score is not a verdict on the islands. It measures how many climate compromises the same calendar month demands.

Watch the handover, not one “rainy season”

Rain becomes most concentrated on the Andaman side in September and October. Phuket reaches 18–19 rainy days and 352–333 mm; Krabi reaches 20–21 days and 329–321 mm. Their overall scores sit at 57–61.

Koh Samui is offset. September still records about 11 rainy days and 181 mm. The sharper fall arrives in October and November: 16 days and 265 mm, then 19 days and 396 mm. This timing agrees with the Thai Meteorological Department's national climate description: rainfall typically peaks earlier on the southern west coast and in November on the southern east coast.

December reverses the July result. Phuket and Krabi fall to roughly seven rainy days and recover to scores of 80 and 79. Koh Samui remains at about 13 rainy days and 253 mm. Its score recovers to 71, but the late wet period has not fully cleared.

Three bookings, three answers

A trip tied to European summer holidays. Start with Koh Samui. July and August retain similarly warm air and sea while cutting long-run rain frequency substantially. This is a relative advantage, not a guarantee of dry weather.

A flexible February or March trip. The coast gap becomes much smaller. February averages roughly three rainy days in Phuket and Krabi and four in Koh Samui. All three are at or near their strongest scores, so beaches, excursions and access can lead the decision.

Late November or December. Reverse the summer logic. Phuket and Krabi are already recovering while Koh Samui's later wet period is still visible. “Thailand's rainy season ends in autumn” is too blunt to plan this trip.

What the averages cannot tell you

A rainy-day count does not reveal the time, duration or local footprint of a shower. Nor does it predict sea state, ferry operations or a specific weather warning. Travellers need local forecasts and operator notices for those questions. The climate series works one level earlier: it prevents three warm-island destinations from being assigned the same seasonal calendar.

Data basis: NASA POWER for the consistent land-climate series, NOAA OISST v2.1 for the sea, and the Thai Meteorological Department's climate description for the west/east-coast timing difference. Reference period 1991–2025; historical monthly averages, not a weather forecast.