Was there during the summer time so can only give my experience at for that particular time.

Generally the weather is very warm and also very humid. Being generally from a relatively cold country it was uncomfortable at first walking around in such humidity. Generally the temperatures always in the 20's (degress celsius). North of the country is a bit cooler than the south so e.g Osaka city would be generally more warm than Tokyo

When it rains, it rains hard and alot and even when it's raining it still stays humid. Got caught out in a freak thunderstorm once, quite literally hopped onto the metro as it was sunny, on it for 10 mins and as leaving it was raining so much and so hard that the streets were already almost flooded from 10 mins of rain.

Basically, if you come to Japan in summertime expect to sweat quite alot and also expect alot of rain at random times.