During the rainy season (May to October), the winds of the southwest monsoon deposit n average rainfall of 50 to 90 inches (1,300 to 2,300 mm), with totals reaching some 160 inches (4,100 mm) on the Bolovens Plateau. The dry season (November to April) is dominated by the northeast monsoon. Minimum temperatures average between 60 and 70 °F (16 and 21 °C) in the cool months of December through February, increasing to highs of more than 90 °F (32 °C) in March and April, just before the start of the rains. In the wet season the average temperature is 80 °F (27 °C).
The monsoon is often followed by a dry period (November-May), beginning with lower relative temperatures and cool breezes created by Asia's northeast monsoon (which bypasses most of Laos), lasting until mid-February. Exceptions to this general pattern include Xieng Khuang, Hua Phan and Phongsali Provinces, which may receive rainfall coming from Vietnam and China during the months of April and May. Rainfall varies substantially according to latitude and altitude; the highlands of Vientiane, Bolikhamsai, Khammuan and eastern Champasak Provinces are wettest.
Temperatures also vary according to altitude. In the humid, low-lying Mekong River valley, temperatures range from 15°C (59°F) to 38°C (100.4°F), while in the mountains of Xieng Khuang the temperature can drop to 0°C (32°F) at night. In Vientiane a minimum temperature of 19 degrees Celsius is to be expected during January. In mountainous areas, however, temperature drops to as low as 14-15 degrees Celsius during the winter months, and during cold nights, can easily reach the freezing point. The average precipitation is highest in Southern Laos, where the Annamite Mountains receive over 3,000 mm. annually. In Vientiane rainfall is about 1,500-2,000 mm., and in the Northern provinces only 1,000-1,500 mm.
Answer is “between November and February”, when it rains least and isn't too hot. This is also the main season for both national and regional bun (festivals). If you're heading up into the mountains, May and July can also be pleasant. Roads can be washed out during rainy season (July to October), but there are plenty of river travel opportunities. Peak tourist months are December to February and during August, although there are relatively few visitors at any time.
Luang Prabang, surrounded by jungle and ‘cut through’ by the flow of the Mekong River, has a typical tropical-monsoon climate and tends to be cooler than destinations further south. The dry season is split into two halves, with the latter months being the hottest. During its ‘coldest’ months (December and January), temperatures can settle at around 17°C and nights and early mornings can be chilly. Once you reach March temperatures start to rise and come April and May it can peak as high as 31°C+.
During the wet season (May to October) you can expect heavy downpours for short periods, most commonly during the night or early morning. Initially this rarely lasts more than an hour or two, and helps to clear the air, leaving bright blue skies in its wake. Come August and September the rainfall can be more prolonged.
The Mekong River
February & March
In recent years water levels on certain stretches of the Mekong River have reached record lows during February and March. This is not simply due to lower rainfall and global warming, but because of eight new Chinese ‘super-dams’ stemming the water flow to ensure a plentiful water supply to regions of China outside the Mekong River’s natural basin. The result of this damming is that the river's fish stocks are declining dramatically in the lower Mekong countries, with other wildlife expected to become quickly over-hunted as a result.
At this time a typically disappointing amount of international pressure is being applied to China to exercise more stringent control on their damming procedure to ensure the countries of the lower Mekong receive acceptable levels of water.