It has become rather cold in Germany since last Wednesday. And as ever so often this phenomenon has been known for a long time. In Germany we call it "Schafskälte" which means something like "sheep's coldness". It's a couple of cold and rainy days after the first period of warm and hot days in the year. We'll enjoy the weekend anyway. And, luckily, in the area around Heidelberg it's never so bad.
See for instance: http://www.bauernregeln.net/schafskaelte.html