If you’re hoping to get lost in the rain on your romantic getaway, take a look at these details about year-round temperatures in Grassau: The hottest months are usually July and August, with an average temperature of 16°C, while the coldest months are February and January, with an average of 1°C. The snowiest months in Grassau are December, January, November and February, with each month seeing an average of 13 cm of snowfall.