Weather in Koh Samui in August

How August compares

August is the 5th warmest month of the year in Koh Samui and ranks 51st of 134 destinations we compare for August - see where else is hot in August. Within Thailand it is 2nd of 6 (Thailand in August). Against London's average high of 21.4°C in August, Koh Samui is 8.0°C warmer by day and 13.1°C warmer at night.

Daytime and night-time in August

The typical August day in Koh Samui peaks at 29.4°C, but that is an average: the warmest 10% of days reach 30.6°C or more. About 100% of days top 25°C and 1 in 4 pass 30°C. Nights average 26.6°C; one night in ten drops to 25.8°C or lower, so pack a layer for evenings. Between 1991 and 2020 the hottest August day here reached 33.0°C (2019-08-06) and the coldest night 24.3°C (2017-08-28).

Rain in August

Wet (more than 8 rainy days): about 19 days with 1 mm or more, 96 mm in total. Year to year it varies a lot - the wettest August was 1998 with 178 mm, the driest 2016 with 39 mm. Humidity averages 78%, which makes the heat feel heavier than the thermometer suggests.

Sea temperature in August

The sea averages 29.2°C - warm enough to swim (22°c+). Depending on the year it ranges from 28.8°C to 29.6°C. For the warmest sea here, come in May (30.5°C).

Sunshine and daylight

Around 1.6 hours of sunshine a day (estimated from cloud cover), 12.3 hours of daylight: the sun rises about 06:12 and sets about 18:38 local time in mid-August. Average cloud cover is 87%.

Is August a good time to visit Koh Samui?

It is a reasonable choice: hot days but expect showers. The best months for Koh Samui on our scoring are February, March, January; months averaging 25°C or more: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec.

Hot days, with little cooling after dark

August feels distinctly hotter than a UK summer: Koh Samui's average daytime high is 8°C above London's. Humidity of 78% adds to the discomfort, particularly if you are active around midday. Loose, breathable clothing makes more sense than anything heavy, and air conditioning is a sensible accommodation priority. Average night lows of 26.6°C mean evenings offer little relief from the warmth. Days reach 30°C about 27% of the time, so the heat is persistent without routinely being extreme. Plan your more active outings around how you feel, leave room for breaks, and do not expect sunset to bring a cool evening.

Rain is frequent; bright sunshine is limited

About 19 days have measurable rain, making wet weather a normal part of an August stay rather than an occasional inconvenience. That does not establish how long rain lasts or when it falls, so do not build plans around an assumed dry morning or a predictable afternoon shower. Sunshine averages only about 1.6 hours within 12.3 hours of daylight: there is plenty of daylight, but relatively little bright sun. A lightweight waterproof and a bag that keeps essentials dry are useful. Keep outdoor plans movable, and choose this month only if cloud and interruptions would not spoil the holiday for you.

Warm water is the strongest reason to go

The sea is a clear advantage in August, averaging 29.2°C, with a year-to-year range of 29°C to 30°C. It suits travellers who dislike a chilly entry, although water this warm may not feel especially refreshing after time in the heat. Temperature alone does not tell you about waves or swimming safety, so follow local advice before getting in. For a long-haul trip from London, the decision is mainly about your priorities: warm swimming conditions are a good reason to consider August; reliable sunshine is not. If your dates are flexible, February, March and January are stronger overall weather choices here.