Thailand’s rainy season is caused by the southwest monsoon that sweeps out of the Indian Ocean with moist air heading in a north-easterly direction across the country, sucked into the void left by rising warm air over the summer Asian continent. The monsoon also coincides with Thailand’s location in the Southeast Asian tropical rain belt – the Inter Tropical Convergence Zone.
The timing of the season isn’t the same around the country and isn’t the same every year although it is reasonably reliable. Chiang Mai does not have the same rainy season as the Gulf of Thailand islands. Koh Samui’s wet season is month’s after the islands on the other side of the Malay Peninsula (the Isthmus of Kra).
The annual celebration of Songkran, the Thai New Year – April 13 – is usually timed to match both the end of the hot season and the start of the annual wet season. But in most provinces the start of the monsoon is usually a month or so later.
The strength and intensity of the rains vary greatly. But, generally, monsoon rains tend to be short, intense bursts of rainfall. They could last for a few hours in the middle of the day, but they could just as easily be over within about 15 minutes in the morning or evening.
SOURCE: https://www.nationthailand.com/news/30404428