Antwerp sits just about 40 miles northeast of the North Sea, and a steady stream of wind off the water tempers its climate year round. Belgium has pretty mild, maritime weather with lots of wet, grey days every year, and Antwerp is no exception. (The country is so small—just 11,787 square miles altogether—that the weather in Antwerp is fairly indistinguishable from that of the rest of the country.)
Your best bet for sunny days is the long stretch from late spring to early autumn, about May to September. The average temperatures in May fall between 65 and 50, while the hottest weather of the year generally hits in July, when the mercury runs between 75 and 55.
These summer months are also the busiest time for tourists, so if you’d prefer a quieter scene, it’s best to wait until September. The weather is still really pleasant—with average highs around 65 and average lows in the 50s—but the crowds have largely faded.
Temperatures begin to dip sharply in the fall, and by November, temperatures range between 50 and 35. January is the coldest month, with an average low of 32 degrees. Snowfall is rare here, though there’s often a lot of chilly moisture in the air, with morning fog a common winter sight.