The climate in Erlangen is very mild, warm and temperate. Rainfall is common all through the year, with even the so-called driest months receiving a major amount of precipitation. The average temperature is a very cool 9 degree Celsius with slight variations.
To see the beautiful nature of Erlangen in all its glory, the spring months of March to May is best time to visit Erlangen. If rain is not a spoiler for you, then consider visiting Erlangen in the Summer months too as you will face less number of tourists.