Best Time to Visit Siem Reap: Monsoon, Light and Heat

Siem Reap runs on two seasons, not four, and the boundary between them is a wind direction. The southwest monsoon delivers 1,209.7 mm of the town's 1,375.8 mm of annual rainfall between May and October — 88% of the year in six months, while December, January and February total 15.5 mm across 15 rain days. Those figures are the climatological normals recorded at the Siem Reap station between 1982 and 2024 by Cambodia's Ministry of Water Resources and Meteorology and published through the World Meteorological Organization.

The calendar decides more than whether you get wet. It sets the water level in the moat around Angkor Wat and across the Tonlé Sap floodplain, moves the point on the horizon where the sun comes up behind the temple by 48 degrees across a year, and governs how many hours of a guided day you can spend on open sandstone. The catalogue we track prices all three without naming any of them.

When exactly are the two seasons in Siem Reap?

The wet season runs May to October and the dry season November to April, with the transition weeks doing the work. May jumps to 143.0 mm from April's 47.5 mm; November drops to 61.8 mm from October's 227.0 mm. Nothing about it is gradual.

The mechanism is the southwest monsoon — the wind reversal that pulls moist air off the Indian Ocean and the Gulf of Thailand across mainland Southeast Asia from roughly May to October, then blows dry from the northeast. Siem Reap sits at 13.36°N on the northern shore plain of the Tonlé Sap, far enough inland that the monsoon arrives as afternoon and overnight rain rather than continuous cloud. September, the wettest month, averages 27 rain days out of 30 — but a rain day is a day on which measurable rain fell, not a day spent indoors.

Siem Reap climate normals, month by month

The table below is the Siem Reap station record, 1982 to 2024, as published by the World Meteorological Organization. Temperatures are monthly means of daily maximum and minimum; rainfall is the monthly total; rain days the mean count of days with measurable rain.

Two numbers there are worth reading twice. September's 305.2 mm is 41 times December's 7.4 mm: the seasonal contrast is not a matter of degree but of climate. And the coolest month by daytime maximum, December at 30.7 °C, sits only 4.6 °C below May's 35.3 °C, while the night gap is wider — January averages 20.0 °C against June's 25.1 °C, a 5.1 °C spread. The dry season's gift is the early morning, not the afternoon. Relative humidity normals are not published for this station in the source we use, so this guide does not quantify them.

What the monsoon changes about what you actually see

Rain is the least of it. The monsoon fills the moat that rings Angkor Wat, refills the causeway ponds that produce the reflected image of the towers, turns the forest around Ta Prohm deep green and raises the Tonlé Sap until the flooded forest is navigable. The dry season empties all of it and replaces green with dust and hard light.

The reflecting ponds are the clearest case. The photograph most visitors arrive expecting — five towers doubled in still water — needs water in the northern pond, which is fed by rain and groundwater, not by a pump. In February, when the station records 3.8 mm across the whole month, it can be low or dry; through September and October, at 305.2 mm and 227.0 mm, it is reliably full. The trade is exact: the months that guarantee the reflection are the months most likely to hide the sun behind cloud.

Southwest monsoon

The wind reversal that carries moist air inland from May to October, accounting for 88% of Siem Reap's annual rainfall; the northeast phase that follows is the dry season.

Flood pulse

The annual rise and fall of the Mekong system that pushes water into the Tonlé Sap through the wet months and drains it from November, governing lake level, fishing and the look of the floodplain.

Chaul Chnam Thmey

Khmer New Year, Cambodia's principal public holiday, observed over three days in mid-April — the hottest stretch before the rains, and heavy with domestic travel.

Bon Om Touk

The Water Festival, the November tradition marking the end of the flood season and the return of the Tonlé Sap river to its downstream direction, held on the full moon of the lunar month of Kadeuk.

Tonlé Sap: the river that reverses, and when it does it

The Tonlé Sap is the largest freshwater lake in Southeast Asia, and its river runs in two directions each year — documented hydrology, not local colour. The lake's surface area grows roughly six-fold at the flood peak, from about 2,500 km² to about 15,000 km², and its volume from about 1.5 km³ to 60–70 km³.

The mechanism is head pressure. During the wet months the Mekong carries so much water past the confluence at Phnom Penh that the Tonlé Sap river cannot discharge into it and instead runs backwards, upstream, into the lake; when the Mekong falls the flow reverts downstream. The Mekong River Commission, the intergovernmental body for the basin, describes that cycle. At the Prek Kdam gauge the annual water-level amplitude measured 7.20 m in 2010–2019, down from 8.27 m in 1962–1972, the reverse phase 112 days against 125, and the reverse flow volume 31.74 km³ against 49.67 km³, according to a 2022 study in Hydrology and Earth System Sciences. The flood is shrinking and arriving later, which makes a boat trip booked early in the wet season less certain than it once was.

For the stilt communities on the floodplain, Kampong Phluk among them, the consequence is visible from the boat. These are villages where people live and work year-round — fishing, drying fish, running schools and shops — and the houses stand on posts six to ten metres tall because the ground beneath is under water for part of every year and dry for the rest. Booked in October you arrive at floor level; booked in March you look up at the same houses from dry ground. Neither version is the real one.

Angkor Wat sunrise is a question of azimuth, not taste

The sun does not rise in the same place twice. Computed for Siem Reap's coordinates, 13.36°N and 103.86°E, with the standard solar-position algorithm, sunrise azimuth swings from 65.6° at the June solstice to 113.9° in December — 48 degrees of horizon, crossing due east around 20–21 March and 23–24 September.

Angkor Wat faces west, which is why the sunrise is watched from the western causeway with the temple in front of you and the sun coming up behind it. Where that sun appears relative to the five towers is a function of the date: in June it clears the horizon well north of the central tower, in December well to the south. Around the equinoxes it rises close to due east, and the alignment of the temple's main axis with the equinox sunrise over the central sanctuary was published by Stencel, Gifford and Morón in Science in 1976, recording the axis at azimuth 270.5°. Accounts since differ on how exact the match is on the equinox date itself, some placing it a day or two either side; the alignment is documented, the precision claimed for it is not settled.

The clock is the calm part of the picture. Sunrise in Siem Reap moves only between 05:36 in early June and 06:32 in late January — 56 minutes across the whole year, because the town sits close to the tropics. A pre-dawn departure is a pre-dawn departure in every month; what changes is where to stand.

Half the reviewed demand in Siem Reap is for one hour of the day — an hour that moves 48 degrees along the horizon between June and December.

Iracema Melo, on reading the Siem Reap catalogue against the solar year

Heat: the practical hour boundary in April and in December

The gap between the hottest and coolest month is smaller than most visitors expect, and it lands on the middle of the day. May averages a daily maximum of 35.3 °C and April 34.8 °C, against 30.7 °C in December and 30.9 °C in January — a spread of 4.6 °C, on a base already hot in every month.

What that spread does to a temple day is disproportionate, because the temples offer little shade and the sandstone holds heat. A circuit through Angkor Thom and Ta Prohm means long walks on open causeways and steep stone stairs. In December a visitor can generally keep walking through late morning; in April the same walk at the same hour is what most itineraries are quietly built to avoid, which is why so many products start before dawn and break in the middle. The source used here publishes monthly means of daily maximum and minimum only, so this guide describes the pattern rather than assigning a temperature to 11:00.

The nights are where the dry season pays off. January's mean minimum of 20.0 °C is 5.1 °C below June's 25.1 °C, and that gap is felt at 05:00 on the causeway, waiting in the dark, more than at any other moment of the trip. Other Khmer sites start from our Cambodia country page.

What the Siem Reap catalogue reveals about the calendar

The shape of the 2,410 experiences we track is a calendar document in disguise. 13 of the 50 products sampled name sunrise or sunset in the title, and they hold 34,427 of the destination's 51,338 reviews — 67%, with the eight sunrise products alone holding 26,112, or 51% of everything reviewed here.

Duration is the second signal. 22 of the 50 products run eight hours or longer, ten at exactly eight hours — the most common length in the catalogue — while only 11 run three hours or less. An eight-hour temple day crosses the hottest window, which is why the calendar and the thermometer are one decision here. Long days also cost more: the eight-hour-plus single-day products carry a median of €46, exactly the catalogue median, against €28.14 for those of three hours or less, inside a spread of €28.80 at the lower quartile to €65.96 at the upper.

Two further readings sit in the same data. 13 products depend on water level — the Tonlé Sap, Kampong Phluk, the Phnom Kulen waterfall — and hold 13,079 reviews, 25.5% of the destination total: a quarter of the reviewed catalogue is priced against a variable the monsoon sets. And in a place whose wettest month averages 27 rain days, 100% of the catalogue cancels free of charge. No product we track carries a weather guarantee or a rain date, which makes free cancellation the only instrument on offer against a washed-out sunrise — in a monsoon climate, the whole of the protection rather than a footnote. 24 of the 50 products are private, and in April the practical value of that is control of the departure hour. Every option sits with today's price on our Siem Reap destination page, and the booking mechanics are set out in our guide to what to do in Siem Reap.

Angkor is a living religious site, in every season

The Angkor Archaeological Park was inscribed on the UNESCO World Heritage List in 1992 and is managed by the APSARA National Authority. It is not a ruin field: monks and lay worshippers use these temples, and shrines inside Angkor Wat receive offerings daily.

The published Angkor Visitor Code of Conduct is short and specific. On dress: "Revealing clothes such as shorts and skirts above the knees and showing bare shoulders are prohibited in sacred places." Photographs of monks require permission, and women should not touch monks. The site has been smoke-free since 2012, and touching carvings or moving artefacts is prohibited outright. Covered shoulders and knees in 35.3 °C is the one rule the hot months make harder to keep, and it is worth packing for rather than improvising at the gate.

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