Thailand can best be described as tropical and humid for the majority of the country during most of the year. The area of Thailand north of Bangkok has a climate determined by three seasons whilst the southern peninsular region of Thailand has only two.
In northern Thailand the seasons are clearly defined. Between November and May the weather is mostly dry, however this is broken up into the periods November to February and March to May. The later of these two periods has the higher relative temperatures as although the northeast monsoon does not directly effect the northern area of Thailand, it does cause cooling breezes from November to February.
The other northern season is from May to November and is dominated by the southwest monsoon, during which time rainfall in the north is at its heaviest.
The southern region of Thailand really has only two seasons -- the wet and the dry. These seasons do not run at the same time on both the east and west side of the peninsular. On the west coast the southwest monsoon brings rain and often heavy storms from April through to October, whilst on the east coast the most rain falls between September and December.
Overall the southern parts of Thailand get by far the most rain with around 2,400 millimetres every year, compared with the central and northern regions of Thailand, both of which get around 1,400 millimetres.