Ko Samui weather in September

In September, Ko Samui sees average daytime highs of 28°C and overnight lows near 27°C, with 140 mm of rain across 22.8 wet days and about 12.1 hours of daylight. It is the 6th-warmest and 4th-wettest month of the year here.

Day-to-day highs hold fairly steady near 28°C through the month. The sky is clear or mostly clear about 6% of the time in September, and the air most often feels oppressive.

Daylight stretches from about 12 h 12 min at the start of September to 11 h 54 min by month's end. Set against the rest of the year, September is Ko Samui's 9th-clearest and 4th-windiest month. For the whole year in context, see Ko Samui's year-round climate.

Daytime high
28°
6th warmest
Overnight low
27°
6th coldest
Rainfall
140 mm
22.8 wet days
Daylight
12 h 06 min
6:13 AM – 6:16 PM
Cloud cover
83%
9th clearest
Wind
18 km/h
4th windiest
Solar energy
5.26 kWh/m²
Travel score
3.9
out of 10

Ko Samui is city in Surat Thani province, Thailand.

Density
219/km²
people per km²
Founded
1956

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Day-to-day highs hold fairly steady near 28°C through the month.

A typical September day in Ko Samui reaches about 28°C in the afternoon and slips back to 27°C overnight — a 2°C spread between the day's warmth and the night's chill. On most days the high lands between 27°C and 29°C. Set against its neighbours, September runs on par with August and on par with October.

24°26°28°30°32°151015202530
Avg high (°C)Avg low (°C)
12827
22827
32827
42827
52827
62827
72827
82827
92827
102827
112827
122827
132827
142827
152827
162827
172827
182827
192827
202827
212826
222826
232826
242826
252826
262826
272826
282826
292826
302826

How September really feels in Ko Samui

Once humidity and wind are folded in, September averages 14 days that feel above 32°C and 0 that feel at or below 0°C — the comfort signal a plain temperature reading hides.

Days feeling ≥ 32°C
14
heat index over the day's high
Days feeling ≤ C
0
wind chill below the day's low

The story in September is heat — humidity pushes the afternoon "feels like" well above the raw temperature.

Ko Samui weather by month

How September compares with the rest of the year at a glance. September is the 6th-warmest and 4th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysDaylight (hours)Travel score
Jan27° / 26°769.411.56.7
Feb27° / 26°355.311.77.3
Mar28° / 27°888.411.96.4
Apr29° / 28°9511.712.25.3
May29° / 27°11621.212.43.8
Jun29° / 27°8918.812.53.9
Jul29° / 27°8418.412.54.2
Aug29° / 27°911812.34.3
Sep28° / 27°14022.812.13.9
Oct28° / 26°2862611.83.9
Nov28° / 26°28920.911.64.6
Dec27° / 26°14513.211.55.9

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 6% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 8% of September, with mostly cloudy or overcast conditions the remaining 85%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
11288
21387
31387
41387
51387
61387
71387
81387
91387
101387
111387
121387
131486
141387
151486
161486
171585
181585
191585
201684
211684
221684
231783
241684
251684
261783
271783
281783
291783
301783

Chance of precipitation in September

Any precipitation

Ko Samui gets around 140 mm of rain over 22.8 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 76% and peaks near 83% on the wettest days. The odds climb toward the end of the month.

0%20%40%60%80%151015202530
Rain chance (%)
168
269
369
470
571
672
772
873
974
1074
1174
1275
1375
1475
1575
1676
1776
1877
1977
2078
2178
2279
2379
2479
2580
2681
2781
2882
2983
3083

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

Most of September feels oppressive — the stacked bands show each day's mix of comfort levels, read off the dew point.

By dew point, September lands in the oppressive band about 71% of days. Sticky, muggy-or-worse air turns up on roughly 100% of them.

0%25%50%75%100%151015202530
Oppressive (%)Miserable (%)
16634
26733
36832
46832
56931
67030
77030
87030
97129
107129
117129
127129
137129
147228
157228
167228
177228
187228
197228
207228
217228
227228
237327
247327
257327
267426
277426
287525
297525
307624

Wind in September

Winds average 18 km/h in September.

Across September, daily wind ranges from about 16 km/h on the calmest days to 18 km/h at its breeziest. It stays fairly steady from day to day.

0510152025151015202530
Wind speed (km/h)Feels
118light breeze
218light breeze
318light breeze
418light breeze
518light breeze
618light breeze
718light breeze
818light breeze
918light breeze
1018light breeze
1118light breeze
1218light breeze
1318light breeze
1418light breeze
1518light breeze
1618light breeze
1718light breeze
1818light breeze
1918light breeze
2018light breeze
2118light breeze
2217light breeze
2317light breeze
2417light breeze
2517light breeze
2617light breeze
2716light breeze
2816light breeze
2916light breeze
3016light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 12 h 12 min at the start of September to 11 h 54 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 18 minutes over September. Sunrise moves from 6:14 AM to 6:12 AM, and sunset from 6:26 PM to 6:07 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.26:146:26
212.26:146:25
312.26:146:24
412.26:146:24
512.16:146:23
612.16:146:22
712.16:146:22
812.16:146:21
912.16:146:21
1012.16:146:20
1112.16:146:19
1212.16:136:19
1312.16:136:18
1412.16:136:17
1512.16:136:17
1612.16:136:16
1712.06:136:15
1812.06:136:15
1912.06:136:14
2012.06:136:14
2112.06:136:13
2212.06:126:12
2312.06:126:12
2412.06:126:11
2512.06:126:10
2612.06:126:10
2712.06:126:09
2811.96:126:09
2911.96:126:08
3011.96:126:07

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 5.26 kWh/m².

For context, solar energy here swings from a low near 4.49 kWh/m² to a high of 6.44 kWh/m² in April across the year — September's 5.26 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
15.4Bright
25.4Bright
35.4Bright
45.4Bright
55.4Bright
65.4Bright
75.3Bright
85.3Bright
95.3Bright
105.3Bright
115.3Bright
125.3Bright
135.3Bright
145.2Bright
155.2Bright
165.2Bright
175.2Bright
185.2Bright
195.2Bright
205.2Bright
215.2Bright
225.1Bright
235.1Bright
245.1Bright
255.1Bright
265.1Bright
275.1Bright
285.0Bright
295.0Bright
305.0Bright
Planning a trip in September

Hazard calendar for Ko Samui

In seasonPeak

In September, Ko Samui sits inside its cyclone and rainy season season. The full year is below, with each hazard's peak months called out.

Ko Samui sits in the North Indian Ocean cyclone zone; the season peaks May and October–November.

Ko Samui's rainy season runs September–December, when most of the year's rain falls.

Cyclone
Rainy season
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is September a good time to visit Ko Samui?

Our two travel scores rate September for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal).

3.9
Sightseeing
9th-best month of the year
4.6
Beach & pool
11th-best month of the year

September is a weaker month for outdoor sightseeing in Ko Samui.

What to pack for Ko Samui in September

From September's temperature range, rain chance, sun and wind — what's worth putting in the bag.

In September, Ko Samui sees daytime highs around 28°C and nights near 27°C. Pack light clothes and rain gear. Worth having: sun protection. About 23 days see rain.

light clothesrain gearsun protection

Sea temperature near Ko Samui in September

The nearest coast averages about 29°C in September — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea offshore is warmest in May at about 30°C and coldest in January near 28°C.

The water stays above 20°C all year — warm enough for a swim any month.

26°28°30°32°JanFebMarAprMayJunJulAugSepOctNovDecwarmtropicalcomfortable to swim
Sea temperature (°C)Comfort
Jan28Warm
Feb28Tropical
Mar29Tropical
Apr30Tropical
May30Tropical
Jun30Tropical
Jul30Tropical
Aug29Tropical
Sep29Tropical
Oct29Tropical
Nov29Tropical
Dec28Warm

UV index in Ko Samui in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 13 (very high — sun protection essential). The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Ko Samui peaks around August 22 at about 14 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 18 at about 10 (very high).

The index reaches 3 or more — the level where the WHO recommends sun protection — every month of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecExtremeVery highAug 22UV 14Dec 18UV 10
UV index (midday)Level
Jan10Very high
Feb12Extreme
Mar13Extreme
Apr13Extreme
May13Extreme
Jun12Extreme
Jul11Extreme
Aug14Extreme
Sep13Extreme
Oct11Extreme
Nov10Extreme
Dec10Very high

Air quality in Ko Samui in September

GoodModeratePoorUnhealthyVery unhealthy

Average fine-particle pollution (PM2.5) through the year — so you can see where September falls, not just the annual average.

Air quality in Ko Samui is worst in January — around 16 µg/m³ PM2.5 (moderate), and holds fairly steady the rest of the year.

Modeled monthly averages (CAMS reanalysis, 2015–2024).

01020JanFebMarAprMayJunJulAugSepOctNovDecModerateGoodJan 2016 µg/m³Nov 1110 µg/m³
PM2.5 (µg/m³)Level
Jan16Moderate
Feb15Moderate
Mar13Moderate
Apr14Moderate
May13Moderate
Jun14Moderate
Jul15Moderate
Aug12Good
Sep12Good
Oct11Good
Nov10Good
Dec12Moderate
Location & terrain

Where is Ko Samui?

Ko Samui sits at 9.5°N, 99.9°E, 8 m above sea level, in Thailand. The nearest listed city is Ko Pha-ngan, 27 km away.

Ban Na SanLang SuanNakhon Si ThammaratPak Phanang

Topography around Ko Samui

How much the land rises and falls, and what covers it, within 3, 16 and 80 km of Ko Samui.

Within 3 km, the terrain around Ko Samui has significant variations in elevation — a maximum elevation change of 476 m around an average of 56 m above sea level; within 16 km, large variations in elevation — a maximum elevation change of 629 m; within 80 km, very significant variations in elevation — a maximum elevation change of 1,358 m.

The land around Ko Samui is covered by trees (51%) and water (42%) within 3 km, water (72%) and trees (24%) within 16 km, and water (82%) within 80 km.

3 km
476 m
16 km
629 m
80 km
1,358 m
treesgrasslandartificial surfaceswater
September weather nearby: Ko Pha-ngan 27 km · Ko Tao 63 km · Surat Thani 79 km · Tha Kham 91 km · Ban Na San 103 km · Lang Suan 104 km · Nakhon Si Thammarat 123 km · Pak Phanang 135 km

Thailand travel facts

Travel advisory
Level 1
Exercise Normal Precautions
Power plugs
Type A / B / C
230 V · 50 Hz
Prices
≈ 31% of US
price level, 2025
GDP per capita
$8,057
2025
Happiness
6.3 / 10
World Happiness, 2025
Homicide rate
4.8
per 100k, 2011
Road deaths
32.2
per 100k/yr, 2019
Income group
Upper middle
World Bank
English
Very low
EF EPI 402
Religion
Buddhist 93%Muslim 5%Christian 1%other 1%

Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.

Data sources

Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Ko Samui (9.5357°, 99.9357°, elevation 8 m), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. Tropical-cyclone seasons are the published climatological basin seasons (NOAA National Hurricane Center / regional warning centres). Sea temperatures are the NOAA OISST v2.1 1991–2020 monthly climatology (public domain), sampled at the nearest ocean point. UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).