History of Nan
Little-known Nan goes back to the depths of the history of Thailand. For centuries it was a separate, autonomous kingdom with few relationships with the outside world.
There is much evidence of prehistoric habitation, but it wasn't until several small mueang united to form Nanthaburi on the Nan river in the mid-14th century - contemporary with the creation of Luang Prabang and the Lan Xang (Million Elephants) kingdom in Laos - that the city became a power to be taken into account. Associated with the mighty Sukhothai kingdom, the mueang took the title Wara Nakhon and played a significant part in the development of early Thai nationalism.
By the end of the -14th century Nan was one of the nine northern Thai-Lao principalities that comprised Lan Na Thai (now Lanna) and the city state flourished throughout the 15th century under the name Chiang Klang (Middle City), a reference to its position roughly midway between Chiang Mai (New City) and Chiang Thong (Golden City, which is today's Luang Prabang). The Burmese took control of the kingdom in 1558 and deported many of the inhabitants to Burma as slaves; the city was completely deserted until western Thailand was retaken from the Burmese in 1786. The local dynasty then regained local sovereignty and it remained semi-autonomous until 1931 when Nan finally accepted full Bangkok dominion. Parts of the old city wall and several early wats dating from the Lanna period can be seen in contemporary Nan. The city of Nan's wats are distinctive: some temple structures show Lanna influence, while others belong to the Thai Lue legacy brought from Xishuangbanna in China, where the Thai Lue people came from.
The seasons are as follows:
November thru February - Dry, "Cool" Season - Winds are predominantly from the north when this season starts. Daytime high temperatures are between 30-35 C (85-95 F), humidity between 50% - 60%. Every day is a sunny day.
This is the best season to travel around Thailand and Nan, ideal to go hiking and trekking, motorbike and city trips, etc.
March thru June - Hot Season - Travellers unaccustomed to the heat should not arrive in this period. Even the Thais have problems in the hottest spells. Temperatures can soar to 40 C (105 F) on the hottest days, and at night the humidity rises. Sometime between late June to late July, evening downpours start to come.
In this season, to get away from the heat you can go swimming in the river or waterfall, or go jungle hiking under the cover of the trees. The roads are dry and good to drive, but protect yourself from the sun and heat.
July thru October - Rainy Season - By "rainy", we are talking about evening downpours. It rarely rains for more than an hour at a time. The mornings are usually sunny, but a thunderstorm or two can come towards the late afternoon or at night. This usually starts in late July or August, but by September there is a thunderstorm on most days, often resulting in flooded streets but cooler night time temperatures. By late September and October, the Nan river is high and there is sometime permanent flooding along the river, sometimes it reaches the Nan Guest House as well (latest in 2010).
The end of the rainy season is the time for the boat races, and ideal for white water rafting. The start of the rainy season is a nice time to travel through the mountains as the hills and rice fields are green (as long as your careful of the slippery roads when it rains);
Sources : Café de Zeven, Nan Guest House, Nan Tourist Office, Wikipedia, GT Rider, Fhu Travel, Nan Lifeway museum