September in Phnom Penh is the rainiest month of the year. In September, the weather in Phnom Penh is typically hot. Average daily high temperature in September is about 32 degrees above zero. On average, Phnom Penh has 28 rainy and 2 cloudy days in September. The rains are usually heavy. September as well as April, May, June, and July is considered the most uncomfortable time to visit Phnom Penh.