For sightseeing trips wear material that is airy and breathy. When I lived in Phnom Penh I would usually wear wide or during the day and also when I went out at night. I tried to avoid going out in the midday sun, but if I had to I would usually cover my skin with long sleeved clothes. But that is just me as I am super pale.
The weather in Cambodia generally falls under two categories: the wet season (May to October) and the dry season (November to April). Lightweight, loose-fitting, cotton clothing is recommended for the dry season, when the weather is hot and humid. Visitors may wish to pack long pants and long-sleeved shirts for hiking, trekking, or outdoor activities. A hat and sunglasses may be useful for when walking around under the sun.
During the rainy season, visitors may want to bring a light rain poncho (plastic ponchos can be purchased cheaply in Cambodia) or a sturdy umbrella. A light jacket or cardigan will come in handy during the months of December and January, when temperatures are at their coolest.
When visiting outdoor temples, including those: Angkor Wat, shorts and T-shirts are acceptable. Shoes and hats are generally removed at the entrance to pagodas. The Silver Pagoda, which is within the Royal Palace grounds. visitors are asked to dress more formally. Gentlemen are required to wear long trousers and ladies should wear long trousers or long skirts and keep their shoulders covered.