Surat Thani Province

Surat Thani Province occupies the upper Gulf of Thailand coast and spills east across a cluster of islands that most visitors know better than the mainland itself. Ko Samui, Ko Pha-ngan, and Ko Tao form the main island chain, separated from the coast by 40 to 80 kilometres of shallow Gulf water. The tidal regime governing all three islands is mixed diurnal — a pattern that confuses travellers accustomed to the reliable twice-a-day cycle of Atlantic or open-Pacific coasts. Through much of the year, the Gulf of Thailand produces one dominant high and one dominant low per day, with a second, much smaller high or low tucked in. Near the equinoxes, the signal becomes more consistently semidiurnal, with two tides of roughly equal height. The pattern rotates through the year with the solar declination cycle, and the difference between a diurnal day and a semidiurnal day can be dramatic: on a purely diurnal day, the water rises once and falls once, and the whole cycle takes 24 hours rather than the usual 12.4. Spring tidal range at Ko Samui runs 2.0 to 2.5 m between high and low water; neap range drops to 0.5 to 0.8 m. During neap tides in May through August, the daily variation can fall to under 0.5 m — barely perceptible to a beach walker, and essentially irrelevant to ferry schedules and snorkelling windows. The Gulf of Thailand is semi-enclosed, open to the South China Sea only through the relatively narrow gap south of the Malay Peninsula, which limits how much Atlantic or Pacific tidal energy can propagate in. The dominant authority for Thailand coastal forecasts is the Thai Meteorological Department (TMD). TMD publishes tide tables, weather forecasts, and monsoon advisories; any activity with a tide-dependent safety component should reference TMD predictions alongside the gridded estimates on this site. The NE monsoon season (November through February) brings the largest wind-generated swell to the east-facing beaches of the island chain. The SW monsoon (May through October) produces rougher conditions on the west side of Koh Samui and along the mainland Surat Thani coast, with ferry services to the islands occasionally suspended during peak southwest winds. Fishing is a significant local industry; squid fishing by boat light is visible at night across the Gulf between the islands, and trawler operations run year-round out of Ban Don Bay on the mainland coast.