Central Thailand experiences the country’s three seasons in equal measure. It's hot from February to June, rainy from June to October, and cool (relatively speaking) from October to January. The one constant is the humidity.
Because of altitude, it is wetter and cooler (and sometimes even a little chilly) in Sangkhlaburi and its surrounding national parks than elsewhere, making this region a comparatively refreshing break from low-lying towns…just be sure to bring an umbrella.
Ayuthaya and Lopburi sit in a wide open plain that receives similar amounts of rain and heat as Bangkok.