May marks the onset of the low season across much of Thailand as large swathes of the country come under the influence of the Southeast monsoon. Humidity can be high in Phuket and West Coast destinations and afternoon downpours can be expected every other day.
Although rainfall is higher between May and October, rainfall is concentrated in short sharp bursts and many visitors to Thailand during these months are pleasantly surprised to experience long sunny days with little rain.
Koh Samui can also receive a little more rain than average in May and temperatures are a couple of degrees higher on average than the preceding months.
Koh Samet is also a great choice for a beach holiday at this time of year, being the driest region of Thailand.
Bangkok and Chiang Mai follow the same season as the West Coast of Thailand.