The only source that I could find immediately is this one: http://www.wettergeschichte-hessen.de/ http://www.wettergeschichte-hessen.de/ Entry 3508 talks about

The only source that I could find immediately is this one: http://www.wettergeschichte-hessen.de/ http://www.wettergeschichte-hessen.de/ Entry 3508 talks about 35 degrees Celsius in May of 1892, which for Germany is freakin' hot. Another entry calls 31 degrees Celsius in June 1818 exceptionally hot, which I would not consider exceptional at all.