Shirakawa-go in August is undeniably hot; a ten-minute walk under the sun will leave your back drenched in sweat.
Sunscreen is a must; it's unbearably hot.
The houses in the Gassho-zukuri style are cool inside, with thick thatched roofs like a quilt, but there are plenty of insects outside, so bring plenty of mosquito repellent.
The village is small; the main street is packed with people at midday, making it difficult to take photos without squeezing in. It cools down in the evening; the path to the observation deck has less shade, but it's a ten-minute climb up, and you don't have to wait for the shuttle bus. The observation deck is truly invigorating.