Broadly speaking, there are two main seasons in Indochina ( Vietnam, Laos and Cambodia ): the rainy season, from May to October or November, and the dry season from December to April. Towards the end of the dry season, in March, temperatures begin to rise, and in some places can top 40°C in April with high humidity. However, it’s a large region with plenty of micro climates, so you’re sure to find pleasant weather somewhere regardless of when you plan to travel. For example: Some cities in Vietnam such as Hanoi , Sapa and Dalat are likely to be cold in the winter, the Mekong Delta in Vietnam can flood which makes travel difficult. Cambodia and Laos have more stable climate and temperature but sometimes the hot day can drain your energy quickly during long day of travelling so prepare necessary thing to protect you from the sun here. 
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