Siem Reap Weather - Cambodia. Siem Reap in Cambodia is characterized by a savanna climate including dry winters during the year. The temperature usually ranges between 21°C and 35°C over the course of a year. During the warm season which is during April and May the temperature can potentially reach above 37°C. April is the hottest month of the year while 19th of April is typically regarded as the hottest day of any year in Siem Reap.