Throughout January you can expect good weather conditions throughout Cambodia, with blue skies, little or no rain and pleasant temperatures (avg temp: 26 °C). Slightly cooler temperatures can be experienced in the more hilly regions of Mondulkiri and Ratanakiri.
This is amongst the most popular months to travel in Cambodia thanks to the temperate climate making long days exploring the Angkor temples far more manageable although lunch time shade is still most welcome.
Along the entire south coast and in the Cardamom and Tatai region you should expect undisturbed blue skies and sunshine. Further north Phnom Penh, Battambang and Siem Reap Province are hot and dry without the crushing heat expected a little later in the year.