Average Temperature :
Average temperature in Tokyo is 16°C (61°F), August being the hottest and January the coldest. There's about 20°C difference between summer and winter. Tokyo has four distinct seasons, mild spring and fall, hot summer, and winter with a few snowfalls. It seems that the hotness in the summer is more a problem than coldness in the winter, for most of the people who come over to Tokyo.
In the summer of 2004, temperature went record high up to 42.7°C (109°F) in the middle of Tokyo (Adachi ward), and days over 30°C (86°F) continued for 40 days. It was hotter in Tokyo than in the subtropical islands of Okinawa.
It was caused not only by the "global warming" going on everywhere around the world, but also by "urban heat island syndrome". Grounds covered with concrete and asphalt keep the sun's heat even during night times, and continuously working air-conditioners and millions of cars produce artificial heat 24 hours a day. It was the wind from Tokyo bay that use to cool down the city, but now the bay area is redeveloped with high and thick buildings, working as a large wall cutting off the wind.
So, if you're not so tough against heat, you might as well avoid summer in Tokyo. Spring or fall, when we have the average temperature would be much more comfortable. Also winter is not so cold (we rarely have temperatures below freezing point) and usually dry and clear. But it's in the summer when we have large and energetic festivals and fireworks all around Japan. Keep in mind that climate is not always the guidepost when to visit.
Tokyo's Average Temperature and RainFall: Today's Temperature:
There's a rain season starting in early June and lasting till mid of July, but showers in the typhoon season (usually in September) are heavier with strong wind, and may well affect public transportation timetables.
But except these typhoons, you don't really have to worry about being soaking wet in Tokyo. A small folding umbrella would do a good job. Tokyo has a very good public transportation system, and you can catch either a bus, a subway or a train almost everywhere, and large key stations usually have wide spreaded underpass with many exits, so you can reach your destination without being wet.
Time to visit:
March to May, or late September to November would be the right season in terms of climate, with mild temparatures and less rainfalls. Blooming flowers like cherry blossoms would be the highlight for the March to May season, where as beautiful autumn colored leaves are the highlights for the September to November season.