Due to the annual monsoon cycles, most Southeast Asia countries have a dry and wet climate with three basic seasons, including Laos. The monsoon or wet season lasts from May to October in which some rainfall will happen suddenly. During this period, the weather is humid and clouds are full of the sky. However, with the well-maintained roads in Laos, it will be fine to bring a raincoat. As to the humidity, make sure you have booked the room with AC. After the monsoon, there is a dry period from November to April, sometimes May even. The country experiences its hottest and very dry weather.
The best time to visit Laos is between November to February when it rains the least and is not too hot. And during this period, most of the Laos’ festival will happen. As for the mountainous northern provinces, it is not bad to go to there in the hot season mainly from March to May. You’d better not go to the southern Laos from March to May for the temperature will break to the 40s in the day-time and nights will be hot too. If you intent to travel by river, rain season will be the best choice.