The weather is rather like New York. Colder in winter, and hotter in Summer, than in the UK. Funnily enough I was looking at an album of pix I took in Tokyo just yesterday, from when I lived there, and there seems a permanent haze in most of them... in summer it gets stifling, the humidity can be a bit mind-boggling. You don't say when you are going, so harder to advise...
Can't say I recall any problem with air-quality.
Japan is 100v, two flat pin plugs (like the US ones). You would be fine running 110v applicances IME.