In Cambodia, the climate is tropical, hot all year round, with a rainy season from May to mid-November, when the south-west monsoon prevails, and a dry season from mid-November to April. The dry season can be divided into two periods: the first is the least hot of the year, while the second, from mid-February to May, before the monsoon arrives, is the hottest of the year. The monsoon withdraws in early November in the north, and between the middle and the end of the month in the center-south.
The climate of Cambodia is fairly uniform because the country is not large. In a typical year, from 1,300 to 1,800 millimeters (51 to 70 inches) of rain fall in inland areas; the rainiest region is the south-west, towards the coast of the Gulf of Thailand, where the rainfall exceeds 2,000 mm (80 in) per year and there are vast areas covered by forest. The climate is a bit cooler in the highlands, such as the Cardamon Mountains, which reach their highest point in Phnom Aural, 1,800 meters (5,900 ft) high, whose slopes are covered by an impenetrable forest and are protected by a nature reserve. The mountainous areas, in addition to being the coolest, are also the rainiest: they receive up to 5,000 millimeters (197 inches) of rain per year, and may experience some showers in the afternoon even before the monsoon season (February to April).
The coolest month in Cambodia is December, while the hottest months are April and May, when the heat is oppressive; in the rainy season, the temperature is a bit lower, but the humidity is higher, so the weather is hot and muggy.
In the capital, Phnom Penh, the daytime temperature is 30 °C (86 °F) in November and December, it begins to rise slightly already in January, and it reaches its highest values in April, with an average of 35 °C (95 °F), but with peaks of 40 °C (104 °F); then it decreases slightly during the long rainy months.
Here are the average temperatures of Phnom Penh.
Phnom Penh - Average temperatures
Min (°C)222324252525252524242322
Max (°C)323335353434333332313030
Min (°F)727375777777777775757372
Max (°F)909195959393919190888686

Throughout the year, 1,400 mm (55 in) of rain fall, with a minimum in January, when it almost never rains, and a maximum in October, with 250 mm (10 in) of rain. The latter part of the monsoon is therefore the wettest period, and has the greatest risk of flooding, especially in the area crossed by the great river, the Mekong.
Here is the average precipitation.
Phnom Penh - Average precipitation

The climate of the ancient capital, Angkor Wat, located in the northwest of the country, is similar to that of Phnom Penh. However, lying in the northern part, the monsoon in Angkor Wat retreats a bit sooner (i.e. in early November), in addition, the winter temperature is slightly lower.
Along the coast, because of the sea thermal inertia, in the pre-monsoon season, it does not get as hot as in the interior, as we can see from the temperatures of Krong Kampot.
Kampot - Average temperatures
Min (°C)212223252524232423232322
Max (°C)313132323231313030313030
Min (°F)707273777775737573737372
Max (°F)888890909088888686888686

Here the rains are more abundant, in fact they amount to almost 2,000 mm (80 in) per year; in addition, the rainiest months are July and August, with more than 300 mm (12 in) per month. Here is the average precipitation.
Kampot - Average precipitation

The amount of sunshine in Cambodia is good from December to February, but also in March and April, despite the first thunderstorms, while it's barely sufficient if not low from June to October because of the cloudiness brought by the monsoon. Here are the average daily sunshine hours in Phnom Penh.
Phnom Penh - Sunshine

Angkor Wat

The sea in Cambodia is warm enough to swim in all year round, as we can see in the following table.
Koh Kong - Sea temperature
Temp (°C)282829303030292929292928
Temp (°F)828284868686848484848482

Tropical cyclones

Cambodia can be hit by typhoons, the tropical cyclones of the western Pacific, though they are quite rare because they usually pass to the north and hit Northern Vietnam and Laos. Moreover, when typhoons affect Cambodia they are generally weakened because they release the bulk of their energy over the coasts of Southern Vietnam. However, they can still bring heavy rains, which can cause flooding. In theory, typhoons can occur between June and December, but generally in Cambodia they pass from mid-October to mid-November, with a maximum at the beginning of November.

Best Time

The best time to visit Cambodia goes from December to mid-February, in fact it is the coolest, and also the driest and sunniest of the year. If you want, you can avoid the first half of December, during which sometimes a typhoon has still affected the country.
The best time for a beach holiday in Cambodia is from December to February, even though on the coast the increase in temperature in the period from March to May is less pronounced than in the interior.

What to pack

All year round, bring lightweight clothing of natural fibers, comfortable shoes, a scarf for the breeze on the coast; from December to March, a sweatshirt for the evening; in the rainy season, a light raincoat or umbrella. For the mountains, spring/autumn clothes, a raincoat or umbrella, a sweater and a jacket for the evening, a bit warmer from December to March.
When visiting temples, it is customary to remove shoes, dress neatly and cover a little.