Thailand July and August weather: Gulf vs Andaman

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Weather month by month

Averages for 1991 to 2020 from the Ko Samui (WMO 48550) weather station, the nearest one to Koh Phangan in the source.

Month	Avg high (°C)	Avg low (°C)	Rain (mm)	Rainy days	Sea (°C)
January	29.0°	24.2°	126.8 mm	11.1	28 °C
February	29.4°	24.9°	64.3 mm	5.6	28 °C
March	30.4°	25.4°	115.8 mm	5.9	29 °C
April	31.8°	26.0°	82.9 mm	8.1	30 °C
May	32.9°	25.7°	131.6 mm	14.0	30 °C
June	32.7°	25.3°	133.6 mm	13.1	30 °C
July	32.3°	25.1°	117.0 mm	14.3	29 °C
August	32.4°	25.1°	102.2 mm	14.5	29 °C
September	31.9°	24.8°	117.5 mm	15.0	29 °C
OctoberRainy season	30.6°	24.3°	294.9 mm	19.3	29 °C
NovemberRainy season	29.7°	24.1°	444.6 mm	18.7	28 °C
DecemberRainy season	29.3°	24.0°	263.1 mm	14.2	28 °C

Sources: ubonmet.tmd.go.th (climate normals), weather2travel.com (sea temperature). As of 4 October 2026.

On the published averages, July and August are clearly drier on the Gulf side than on Phuket’s Andaman coast. Wikipedia’s Ko Samui page lists 117.0 mm in July and 102.2 mm in August for the Ko Samui station, against 258.2 mm and 286.8 mm for Mueang Phuket on its Phuket province page. Air and sea temperatures are almost the same on both sides, so rainfall is what separates them.

Koh Phangan has no weather station in our sources. We use the Ko Samui station as the nearest reference, because Koh Samui lies in the Gulf of Thailand with Koh Phangan to its north. Treat it as a guide to the region, not as a measurement taken on Koh Phangan.

The stations and periods used

The figures on this page come from several sources, and their periods differ. Where they disagree, we give both values and name the source.

• Ko Samui station: the climate table on Wikipedia’s Ko Samui page covers 1991 to 2020 (sunshine hours 1981 to 2010). Its sources are the World Meteorological Organization and the Royal Irrigation Department. The monthly table above uses the Thai Meteorological Department normals for the same station and period. Its rainfall and temperatures match the Wikipedia table, but it counts rainy days differently (see Rainy days).
• Phuket: Wikipedia’s Phuket province page gives two stations for 1981 to 2010, Mueang Phuket district and Phuket International Airport, with temperature and rainfall from the Thai Meteorological Department.
• Both places: Weather2travel publishes long-term monthly averages for Koh Samui and Phuket, processed from CRU (University of East Anglia), Met Office and Netherlands Meteorological Institute data. It does not state a period.

Because the periods differ, compare the stations within one source before comparing across sources.

July and August side by side

For Koh Samui temperatures, rainfall and sea temperature by month, see the table above. Weather2travel, which covers both places with one method, gives the same values for Koh Samui and Phuket in July and August: a daytime maximum of 31 °C, a night-time minimum of 24 °C, sea temperature of 29 °C and 6 hours of sunshine a day. The station normals in the table above are about a degree warmer than Weather2travel’s figures, for example 32.3 °C against 31 °C for the July daytime high.

The real differences are in rain. Weather2travel’s Phuket page lists 284 mm in July and 265 mm in August, and it also lists less rain for Koh Samui. The next sections use station figures to show how large the gap is.

Rainfall

For the Ko Samui station (1991 to 2020), Wikipedia lists 117.0 mm in July and 102.2 mm in August, about 219 mm for the two months. For Mueang Phuket (1981 to 2010) it lists 258.2 mm and 286.8 mm, about 545 mm, roughly two and a half times as much. At Phuket International Airport the figures are 261.5 mm and 329.8 mm.

Phuket’s rain peaks in September, with 361.2 mm at Mueang Phuket, and October is still wetter than August, with 320.1 mm. On Koh Samui the peak comes later, as described below.

Rainy days

The number of rainy days depends on how a day is counted, so the sources give different values. Compare the places within one counting method.

• Weather2travel counts days with some rain, with one method for both places: 17 in both July and August for Koh Samui, and 20 and 19 for Phuket.
• The Thai Meteorological Department counts a rainy day from 0.1 mm. Its 1991 to 2020 normals, used in the monthly table above, give 14.3 days in July and 14.5 in August for the Ko Samui station. Wikipedia’s Phuket province page, based on the same department’s data for 1981 to 2010, gives 19.6 and 19.0 for Mueang Phuket and 20.3 and 20.2 for Phuket International Airport. The periods differ.
• The Wikipedia table for Ko Samui, based on World Meteorological Organization data, counts only days with at least 1.0 mm: 10.7 and 10.5. The Phuket tables on Wikipedia give no such count, so this figure has no Phuket counterpart.

Both comparisons point the same way: Phuket has more rainy days than Koh Samui in July and August.

Sunshine

Sunshine is the measure on which the two coasts are closest.

• Weather2travel gives 6 hours a day in both months at both places.
• Wikipedia gives 155.0 hours in July and 151.9 hours in August for the Ko Samui station (about 5.0 and 4.9 hours a day). Mueang Phuket has 151.9 hours in both months (4.9 hours a day).
• Phuket International Airport is lower in August: 114.7 hours, or 3.7 hours a day.

Daily sunshine of about 5 to 6 hours is a monthly average, not a promise for any given day. July and August are not sunnier on one coast than the other in any large way, apart from the Phuket airport August figure.

Sea temperature

Weather2travel lists 29 °C in July and August for both Koh Samui and Phuket. The same source shows 30 °C for both places in May and June, and 28 °C for Koh Samui in November, so the sea stays warm through the year. For a family or group, sea temperature is not a reason to choose one coast over the other.

Tropical storm risk is listed as 0 per cent of the month for July and August at both places. Weather2travel shows a small share only later in the year: for Koh Samui 1.2 per cent in October, 2.2 per cent in November and under 1 per cent in December, and for Phuket 1.9 per cent in November.

What the sources say about the southwest monsoon

Wikipedia’s Geography of Thailand describes the rainy season, also called the southwest monsoon, as running from mid-May to mid-October. Moist air moves from the Indian Ocean towards Thailand and brings abundant rain. The same article says certain areas on the windward sides of mountains, such as Ranong province on the west coast of southern Thailand, receive more than 4,500 mm of rain a year.

The article also notes that the Andaman Sea is a marginal sea of the northeastern Indian Ocean, while the Gulf of Thailand is a northern extension of the South China Sea. On Phuket, a mountain range runs from north to south along the west side of the island, according to the Phuket province page. The sources do not explain directly why Phuket is wetter than Koh Samui in July and August, so we do not claim a mechanism here.

The seasonal calendars differ as well. The Geography of Thailand page says that in southern Thailand the west coast peaks in September and the east coast from November to January. At Mueang Phuket, the monthly rainfall on the Phuket province page stays below 75 mm from December to March and rises above 140 mm from April to November.

The Gulf side is not dry all year

July and August are the quieter months for the Gulf. The wet peak there comes later, in autumn, as our guide to Koh Phangan weather by month shows. In the 1991 to 2020 normals for the Ko Samui station, October brings 294.9 mm, November 444.6 mm and December 263.1 mm. Those three months total 1,002.6 mm, about half of the annual 1,994.3 mm.

If your holiday is in July or August, you are travelling before that peak on the Gulf side. On the Andaman side you are in the wet season, with September and October still wetter at Mueang Phuket.

The verdict for July and August

For a family or group travelling in July or August, the Gulf of Thailand islands, including Koh Phangan, are the drier choice on the published figures. Rainfall at the Ko Samui station is less than half that at Mueang Phuket, and rainy days are fewer, though not few. Sunshine, air temperature and sea temperature are close to equal on both coasts.

Neither coast is dry. Weather2travel counts rain on 17 days in each of the two months for Koh Samui, so plan outdoor time around showers.

Our sources give no wave heights, boat cancellation rates or beach flag conditions. For sea conditions on your travel dates, check the ferry or boat operator and the local forecast close to departure.

How we checked

We compared the climate tables for Koh Samui and Phuket on Weather2travel and Wikipedia, the Thai Meteorological Department normals for the Ko Samui station, and the description of the monsoon seasons on the Geography of Thailand page, all read on 4 October 2026. Where sources give different values, both are stated above with the source named.