Phnom Penh’s climate is generally tropical so that, it tends to be hot year-round with minor variations. The average temperature is between 28 oC and 33oC. Indochina travel Cambodia
Phnom Penh has four seasons in a year: cool and dry from November to February; hot and dry from March to May; hot and wet between June and August; and cool and wet from September to early November.
The best time to visit in Phnom Penh is between November and February, when the weather is cool and dry. The cool season (November–February) is the peak time for tourism – mild enough to discover the temples in comfort but warm enough to sunbath. Humidity and temperatures increase slightly during the hot season (March–May), with Phnom Penh and Battambang seeing peak daytime temperatures of 33–35°C. Cambodia travel tours
As February turns into March, the heat begins to build until April and May, when it can be quite stifling and uncomfortable. If traveling at this time, it is necessary to keep well in the shade as much as possible, wear sunglasses and caps, and bring a bottle of water. The heat can be very unforgiving especially for those unaccustomed to this kind of weather. Most hotels and restaurants are well-cooled, either with air conditioners or with fans. However, not all public or private transportation is so accommodating.
In May and June, the rains come and bring a measure of relief. It rarely rains all day, and mostly it will rain for an hour or two hours in the afternoon or evening. By early September, the heat begins to decrease and the evenings are fresher. Through October, the frequency of the showers slows and the humidity begins to lower. Though the wet season is low season for tourism, visiting at that time does have its advantages.
The rainy, monsoon season consists of varying numbers of rainfall. Nearly all rain falls as showers; sometimes, it is difficult to value all the sights in and around the city as the rain is extreme and many streets are flooded and muddy. Thankfully the showers go quickly; but you might be unlucky and have a few days of continuous rain; however that is more likely to occur in May and September.