July is when the rain arrives in earnest through the bulk of the country and hiking through north Thailand isn’t recommend unless you’re a mud-lover, although the countryside will have greened up, rice planting is in full swing and the waterfalls and rivers are in full flow.
Elsewhere Bangkok and the central region will experience daily heavy showers, but usually in the afternoon. The best bet is to head to the east coast beaches on the mainland around Hua Hin, Cha Am or Khanom or out to the islands of Koh Samui and Koh Phangan. Perfect dive conditions can be enjoyed here. Prices rise steeply when school holidays start.