If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in Slotermeer: The hottest months are usually July and August with an average temp of 61°F, while the coldest months are January and February with an average of 38°F. The rainiest months in Slotermeer are October, September, November, and December, with each month seeing an average of 9 inches of rainfall.