If you plan on visiting Japan in the summer, unless you plan on staying up north like in Hokkaido or Aomori, you can expect temperatures to be in the mid-30s C here for most of the summer and even into autumn in some parts.
I’ve been to the following two major cities in Japan during the summer, and I have a few tips.
Tokyo: With so many streets lined with buildings rather than trees, it’s a concrete oven in the summer.
To try and survive, I recommend getting some items that will help cool you down from an 100-yen shop like Daiso (they have neck scarves you douse in water that cool your neck and fans). Also, feel free to roam department stores or electronics stores for no other reason than to bask in their AC.
Kyoto: This is like Florida in a heat wave. The heat just hits your lungs like you’ve run into a wall. This is all because Kyoto is in a valley surrounded by mountains, making it a little bowl that lets heat and humidity just sit there. At least Tokyo is near Tokyo Bay in some areas, allowing for a sea breeze every now and then.
If at all possible, try to avoid Kyoto in the summer. Kyoto is the place to go in the autumn and spring. If you can’t avoid it, however, I survived with those 100-yen items mentioned above and by making a beeline from one convenience store to another. I used to love admiring the ice cream section of each store for no other reason than it’s the coldest section of the store.
Most temples and shrines, unfortunately, don’t have AC, which means your risk of heat stroke greatly rises. I have found the river near Arashiyama can be a nice respite from the heat, at least, but Kyoto will suck the soul out of you if you stay outside for too long in the summer.