Weather in Siem Reap in September

A hard month. September asks something of you. Go in knowing what. You should expect rain on 25 days of the month, which is more days wet than dry.

September ranks 8 of 12 in Siem Reap: past the worst of the year but short of December, which is the month to book if the dates are open.

Rain on 25 of the month's 30 days, 8 of them heavy. At that frequency a wet-weather backup is not a contingency, it is the itinerary — book the indoor things first and treat the clear days as the bonus. Natural fibres and a second shirt for the day. At 87% nothing dries, including you.

What the averages hide

Averages flatten a month, and this one hides a few things: 18 days top 30 °C — hot spells arrive, but they break; the day barely swings — 6 °C between the overnight low and the afternoon high. Across the twenty years measured, the hottest September day reached 36 °C and the coldest night fell to 21.9 °C, and the wettest single day dropped 81 mm — roughly 35% of the month's rain in one day.

September is statistically steady but not small: the year-to-year variation is modest in relative terms, yet the wettest September on record still delivered 362 mm against 101 mm in the driest. In a month this wet, even a tame spread is a lot of rain in absolute terms — the 231 mm average is a reasonable planning figure, not a promise.

Wind is light at 5.2 km/h, so the temperature on the page is close to the temperature on your skin, and humidity at 87% is the real story though — it is the number that decides how the day feels, not the 30.4 °C.

How September sits in the year

Temperatures hold steady either side: 30.7 °C in August, 30.4 here, 31 in October. Nothing about the timing within the month matters much. Rain is the thing that moves: October brings 7 days less of it, so pushing the dates later buys drier weather.

Daylight in September

Siem Reap sits close enough to the equator that daylight barely moves all year — 12.2 hours in September, against a range of just 1.6 hours across the whole calendar. Sunrise and sunset land at roughly the same time whenever you come.

An inland climate

Despite being inland, the year stays fairly even: 16.1 °C between the year's warmest afternoons and coldest nights.

How busy it gets

Quiet. Few other visitors, cheaper rooms, and a real chance some seasonal operators are shut.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Siem Reap versus nearby

If Siem Reap is one option among several, the nearby comparison is worth making. Ubon Ratchathani is close enough to be a coin toss — within a degree on temperature, 1 day less rain. Nakhon Ratchasima is close enough to be a coin toss — within a degree on temperature, 1 day less rain.

So: September is a compromise. December is 29 points better and is what Siem Reap is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in Siem Reap

See all twelve months in Siem Reap, or where else to go in September.