Thailand forum

As usual, Somtam is absolutely correct about what weather to expect in Thailand during the period you mention. I think if you look at when farmers plant and harvest crops, whichever country you visit, you get a good idea of what to expect weatherwise. In Thailand, fields are prepared for planting new crops of rice and other produce in March (weather is hot/sunny/very dry/little humidity) after the Thai New Year (Songkran, mid April) crops are sown and seeds readily germinate as there is a mix of sunshine and high humidity with more frequent rainfall.

This should not put you off visiting Thailand in May. You would get several hours of sunshine each day in most provinces but be prepared for rain showers. The rain, however, is warm and usually a welcome relief from the build up of it. Find a few good places to chill out and you won't even notice it's a tad wet.