A common misconception is that monsoon refers to rain, but in fact monsoon refers to the wind. The word, ‘monsoon’, comes from the Arabic word, ‘mausim’, meaning season and nowadays monsoon is the term used for the seasonal shift in wind direction which usually brings on a different season of weather.
South-West monsoon in Thailand
The south-west monsoon usually blows in from the Indian Ocean from June-September, but it can sometimes start early in May. The intensity and duration varies from year to year, but in Thailand it is the Andaman Coast (Phuket, Krabi etc.) that feels the effects the most. In simplistic terms, the heat of March-May brings about a significant difference between land and sea temperatures. This causes hot air to rise creating low pressure which in turn causes the monsoon wind to blow towards land. As the wind blows it attracts moisture from over the oceans which may fall as rainfall on the mainland. Coming in from a south-westerly direction, it is the Andaman Coast of Thailand which is hit the hardest. Downpours can be torrential and may be accompanied by gusty winds and high waves. Some boat services are temporarily suspended during May-October on the Andaman Coast, but it should be noted that it doesn’t rain all day, every day. There is still plenty of sunshine and temperatures remain high.
Although the period from June-September sees more rain on the Andaman Coast, the Gulf Coast (e.g. Ko Samui, Ko Pha Ngan) and the East Coast (Pattaya, Ko Chang) don’t escape completely, but there is still less rainfall here than the Andaman Coast.
North-East monsoon in Thailand
The north-easterly wind brings cooler temperatures and generally less humidity and less chance of rainfall for most of Thailand including the North (e.g. Chiang Mai, Chiang Rai), the North-East, Bangkok and the East Coast.
However, the north-east monsoon has a slightly different effect on the more exposed Gulf Coast of the southern peninsula of Thailand (e.g. Ko Samui and nearby islands). As the wind blows from the north-east it travels across the Gulf of Thailand picking up moisture. This usually means that Ko Samui and offshore islands can receive significant rainfall between November and January.
March to mid-May
The transitional period from the north-east monsoon to the south-west monsoon can result in extremely high temperatures throughout Thailand, particularly in the north and north-east of the country.