Late July - how bad is the heat?

I have spontaneously booked a trip to Japan during the last week of July! I'm going to be in Tokyo for a couple of weeks.
Now I am really starting to worry about the heat, though. Is sightseeing even an option, or should I only plan for spending my days in shoppingmalls with AC?

I have been to Japan before in early September, and that was ok, but I hope it won't be MUCH more humid and hot than that...

Tokyo will be hot so I would suggest getting out of the city and head for the country side or head north. When you go out take a few bottles of water and keep hydrated.

If you compare to early september last year, it will be just about the same. (September 2008 was warmer than usual)

I'll try to get out of the city when I can. :)

Yes, I was referring to last September. If does not get much worse than that, I'll survive. :)

Just wondering, as I'm going around the same time (late July-late August): how "hot" is hot? What's the average temperature (in Celsius) around this time (in Tokyo)?

The average daytime high in Tokyo is 30-31 degrees in August. The actual daytime temperature typically varies by plus/minus five degrees. And then, there is a high humidity, which makes it feel much hotter than the temperature looks like.

that is mighty hot! I am not really the type to wear shorts. I was only planning on bringing one pair along. Should I buy more? (Oh btw, I will be in Japan 8-25th July).

Actually Japanese people are not big shorts wearers either, but temps in late July in Tokyo can be in the high 30s, and that is even more likely in Kyoto and Osaka. If you must wear trousers, expect to really sweat a lot. Drink plenty of cold water and use a fan and a small towel. Also make sure the trousers are a dark colour so the sweat won't show.

If it were not for the tens of thousands of concrete buildings in and around Tokyo-to, midsummer in Tokyo would be rather pleasant. However, all that concrete mass stores heat pretty well so that even at night Tokyo can be rather warm. Think of why deserts tend to have wider temperature variations than forests at the same latitude.

Here is a wikipedia article:
http://en.wikipedia.org/wiki/Urban_heat_island

Unless you like muggy daytime conditions, head to the hills.