Laos has three distinct seasons. The hot season is from March to May, when temperatures can soar as high as 40°C. The slightly cooler wet season is from May to October, when temperatures are around 30°C, tropical downpours are frequent (especially July-August), and some years the Mekong floods.
The dry season from November to March, which has low rainfall and temperatures as low as 15°C (or even to zero in the mountains at night), is "high season" (when the most tourists are in the country). However, towards the end of the dry season, the northern parts of Laos — basically everything north of Luang Prabang — can become very hazy due to farmers burning fields and fires in the forests.