Temperatures drop slightly throughout Thailand in July (avg temp: 28 - 29°C), and as the month progresses, rainfall will increase with short daily showers likely to become more prolonged as you move towards August, and some of the heaviest rains of the year.
In Chiang Rai and the Golden Triangle region, heavy rains will make trekking hard work; however the countryside is more green and lush, and the rivers and waterfalls are in full flow.
In terms of beaches, the islands of Koh Chang and Koh Kood are particularly wet in July. Further south, rough seas may be experienced off the west coast and the beaches of Phuket, Krabi and Khao Lak have a much higher chance of heavy downpours than the east coast beaches of Koh Samui, Khanom and Koh Pha Ngan, which are significantly drier and are still enjoying prolonged sunny spells. July is a particularly popular month for diving around Koh Tao.
sun and sand?
Hua Hin, Pranburi, Cha Am, Koh Samui, Koh Pha Ngan, Khanom