Map from Google - Laos

The climate of Laos is tropical in most of the plains, while it's subtropical in the far north and in the mountains, besides it's governed by the monsoons: the winter monsoon, which brings good weather and a little cool at night from November to February, and the summer monsoon, which brings humidity and rains from May to the beginning or the middle of October. However, the monsoons are quite irregular: they do not always arrive in the same period, and they don't always have the same intensity.
The period March to May, before the monsoon, is the hottest of the year, and it is still dry, but with some thunderstorms in the afternoon; the summer months are less hot, with highs around 30/32 °C (86/90 °F), but with higher moisture. April is the hottest month, with average highs about 35 °C (95 °F), but with peaks of 40/42 °C (104/108 °F) in the centre-south and even 45 °C (113 °F) in the north, which has a greater continentality. In May, the temperature begins to decrease in the south, while in the north, where the monsoon arrives later, it's still very hot, especially in the first part.
During winter, nights are cool in the north, around 13/15 °C (55/59 °F) from December to February in Luang Prabang, but the temperature can drop almost to the freezing point; highs are around 27/28 °C (81/82 °F) in December and January, while as early as in February they reach 31 °C (88 °F). Sometimes during the day it can get very hot even in winter.
Here are the average temperatures of Luang Prabang.
Average temperatures - Luang Prabang
Luang PrabangJanFebMarAprMayJunJulAugSepOctNovDec
Luang PrabangJFMAMJJASOND
Min (°C)141518212324242423211814
Max (°C)283134353433323132312927
Min (°F)575964707375757573706457
Max (°F)828893959391908890888481

In the centre-south, the winter temperatures are slightly higher (see Vientiane, Savannakhet), and outbreaks of cold air are rarer and less intense.
Here are the average temperatures of the capital Vientiane.
Average temperatures - Vientiane
VientianeJanFebMarAprMayJunJulAugSepOctNovDec
VientianeJFMAMJJASOND
Min (°C)161922242525252524231917
Max (°C)283033343332313131313028
Min (°F)616672757777777775736663
Max (°F)828691939190888888888682

In the far south (see Pakxe) winter is even warmer, so that normally highs are around 30/31 °C (86/88 °F) even in December and January.
Average temperatures - Pakxe
PakxeJanFebMarAprMayJunJulAugSepOctNovDec
PakxeJFMAMJJASOND
Min (°C)182124252525242424232119
Max (°C)313335353331313030313130
Min (°F)647075777777757575737066
Max (°F)889195959188888686888886

The rainfall amount is normally between 1,400 and 2,200 millimetres (55 and 87 inches) per year in the plains, and may exceed 3,000 mm (118 in) in the most exposed mountain slopes. The rainiest months are usually August and September, which correspond to the final period of the monsoon, also because of typhoons (see below), which bring an additional amount of rain.
Here is the average precipitation in Vientiane.
Average precipitation - Vientiane
VientianeJanFebMarAprMayJunJulAugSepOctNovDecYear
VientianeJFMAMJJASONDY
Prec. (mm)8133585245280270335295801031661
Prec.(in)0.30.51.43.39.61110.613.211.63.10.40.165.4
Days12481518202117921118

The amount of sunshine is good in the centre-south (see Savannakhet and Vientiane) in the dry season, from November to April, while in the north (see Luang Prabang) it's never good because even in the dry season the sky is often cloudy; in the extreme north, in winter the weather is often dreary and there can be some drizzle; sometimes this can happen even in Luang Prabang and the rest of the north. During the period of the summer monsoon, the cloudiness prevails throughout the country, although sometimes the sun can come out.
Laos, a landlocked sea, is mountainous and covered by forests, and it's home to several parks and protected areas. In general, the climate becomes milder with increasing altitude, but it becomes even more rainy, with the first pre-monsoon showers more frequent than in the plains, and definitely abundant rains in the monsoon period.
Here are the mean temperatures of Bounneua, located in the far north, at 900 metres (3,000 feet) above sea level.
Average temperatures - Bounneua
BounneuaJanFebMarAprMayJunJulAugSepOctNovDec
BounneuaJFMAMJJASOND
Min (°C)101012161921212019171410
Max (°C)222527292928282828262422
Min (°F)505054616670706866635750
Max (°F)727781848482828282797572

At high altitudes, it can get cold even during the day in the winter months, especially in the north. The highest peak is Phou Bia, 2,819 metres high.

Laos

From June to November, but more often from August to October, Laos can be reached by typhoons, the tropical cyclones of south-east Asia, which come from the South China Sea, and affect more directly the coast of Vietnam, though they can bring wind and heavy rains also in Laos. Typhoons, as well as waves of bad weather during the monsoon season, may cause flooding and especially the overflowing of the great river that runs through the country from north to south, the Mekong.

When to go

The best time to visit Laos is from November to February, being the coolest part of the dry season. In November, however, typhoons or tropical storms may still pass through the country, and sometimes even in the first half of December. In February, usually the temperature begins to rise, so that it can exceed 35 °C (95 °F); therefore the best time is from mid-December to late January, even though sometimes it can get hot even in January, but the humidity is not high and the nights remain cool. It should be remembered that in winter it can get cold at night across the country, so that the minimum can drop below 10 °C (50 °F), but especially in the north, where it can drop in some cases even to the freezing point.
In March and April, and even in May until the arrival of the monsoon, it's uncomfortably hot in the lowlands, while in the mountains it's more bearable. The weather is still good, although during the afternoon there may be some showers and thunderstorms.
During the rainy season, from May to September, the heat is sweltering, at least in the lowlands, the sky is often cloudy, and sometimes torrential rains can make travelling difficult; in the brief periods of good weather, which last a few days, the heat returns to be intense.

What to pack

In winter: in the north (see Luang Prabang), light clothes for the day, a jacket and a sweater for the evening, raincoat for the drizzle; in the northern cities above 1,000 metres (3,300 feet), spring/autumn clothes, a warm jacket and a sweater for the evening. For the south-central plains (Vientiane, Savannakhet, Pakxe), light clothing, a light sweatshirt for the evening, possibly a light jacket or a sweater for cooler evenings. For the highest peaks, warm clothes, down jacket, hat, gloves.
In summer: all over the country, at low altitude, lightweight clothing of natural fibres, light raincoat or umbrella, a light sweater for the evening; above 1,000 metres (3,300 feet) you can add a sweatshirt and a light jacket; for the highest peaks, warm clothes, sweater, raincoat, hiking shoes.
To enter pagodas, it is customary to remove shoes, dress neatly and cover a little.