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 Onsala: 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 2°C. The snowiest months in Onsala are January, December, February and November, with each month seeing an average of 25 cm of snowfall.