|The weather of Northern Thailand, including Chiang Mai, differs from the normal wesern versions of seasons like spring, summer, autumn and winter. Instead, the region has three distinct seasons: the cool season, hot season and rainy season. Chiang Mai is known as the 'cool capital', andif compared to the dirty sticky heat of Bangkok, the climate is far more pleasant.
The most popular time for visiting Chiang Mai ( weather wise) is the cool season, which runs from December to the end of February. It is in fact pleasantly chilly in the evenings, and if you are planning on visiting Chiang Mai at this time of year, it would be wise to bring some all weather gear along. Particularly if you intend hiring a motorcycle to get around, or going on a mountain trek, pack some warm kit. However, don't leave out your summer clothes, as midday temperatures can climb well into the 30s (Celsius).
The weather start heating up in Chiang Mai after New Year, and by mid-March, the nippy nights of February are nothing but a pleasant memory, with daytime maximums regularly reaching 40C. Try to avoid a holiday to Thailand during this time (April to June) - unless you are completely accustomed to tropical heat, you are likely to find the humidity utterly draining. Despite all the moisture in the air, however, there is virtually no rain during this period.
With the lack of water, blazing heat and slash-and-burn agricultural practices, the usually lush green jungle that covers the city's surrounding hills turns to a charred brown from fires that burn almost constantly from January until the rains arrive. Not only does this affect the vegetation, but a trademark 'hot season haze' hangs over the entire city and its surrounds, obscuring the otherwise beautiful vistas.
The southwest monsoon usually arrives from India at the end of May, and from then until November the weather in Chiang Mai and Northern Thailand get very, very wet. The rainy season is characterized by torrential downpours, but they tend to be sudden bursts that only last for an hour or so, rather than a steady stream of water. Although mosquitoes are rife during this time, the rainy season is otherwise a pleasant time to visit the north. The rains bring respite from the heat, and the landscape returns to its strikingly gorgeous shade of green. Rainfall is usually heaviest in September, with an average precipitation of 250mm for that month.
Overall, the weather of Northern Thailand is far more temperate than central or Southern Thailand. The area is more than 2000kms from the equator and much closer to the Tropic of Cancer. This combined with its mountainous terrain and location in the Asian interior brings cooler temperatures and less humidity. However there is still quite a bit of precipitation during the rainy season and the weather gets very chilly once you leave Chiang Mai and head up into the mountains. Don't forget your umbrella!