Your Japanses jacket isn't linedc in silk, by any chance? Silk will be breathable and can help with keeping cool in the heat. The hottest I've ever managed to wear leather in was 22C, but that invovledc a lot of sheer bloody-mindedness to manage it.... Summer here in London is usually anywhere between 26C and 34C. It's hateful. Money no object I'd spend July and August in a cold climate, and only return to London for the Autumn. In my perfect world, there'd be two weeks of 22C weather in July (because I like to wear linen sometimes), but otherwise it would never rise above 15C.
Yea, I agree. 15C is about the perfect temperature because you can wear jackets without sweating (or even 10C!). And no high humidity! I'm starting to hate humid weather more and more nowadays for some reason. Yesterday I went out and I could smell the humidity in the air, I hated being outside and went back inside as soon as I could only to turn up heating bit more and strip down to a T-shirt. Anything over 20 is starting to become annoying because you're starting to have problems with choosing clothing or sweating, especially if you have to walk somewhere. Unless you're a slow walker.