Basically, Thailand has three seasons: a dry and hot season (from November until February), a very hot season (from March until the end of April; the weather is still dry, but very hot, and doing sightseeing tours may be uncomfortable) and a wet and hot season (this is the monsoon season from May to the end of October), and these distinctions also apply to the northern part of the country. In the North, however, the weather is more temperate and most of the time really pleasant. It is always some degrees cooler than Bangkok or the South; and in the hills you may even need a cardigan or a sweater. The months from November to April are all good for travelling in the North.
 

Clothing

You should dress for a tropical climate: cotton or silk clothes are most comfortable (linen is too heavy). Exposing too much of your skin will not only earn you a sunburn, but is also frowned at by the locals. Not matter, how hot is is, gentlemen must wear long pants in urban areas. For ladies, pants are also o.k., or you can wear a skirt or a dress that covers the knee. Sleeveless shirts or sweaters should also be avoided.  The rules are are even more strict rules for temple visits.