We go to Thailand several times a year and we're always there for awhile in July-August.
At that time Bangkok can be overcast during the day and maybe with some rain (which doesn't last long). Temperatures are still warm.

We were in Phuket a few years ago mid-August and although there had been rain up till the day we arrived, we didn't see any rain at all in ten days (none at night either). It was hot and sunny the whole time - good weather for beach or pool.
Maybe we were just lucky or maybe it was because it was at the end of August rather than the start.