Weather
The normal day temperature in winter would be from -10C up to +10C - you never really know what to expect. Same in summer ... we had summers with only 20C but sometimes also 40C.... Better check the internet for a forecast before you go - they are normally right.
Oh and .. if you come for CeBIT. This is normally the time when we have the first nice spring weather in the year - in 2001 it didn't work though!!!
The picture shows an outdoor ice rink at Steintor in winter 2001/2002.

The best starter
Part of Herrenhausen
Taking breath
Hannover, Gemany, Old Town