Just had a quick look online and this came up :-
The climate of Belgium is cool and wet because it is influenced by the Atlantic Ocean, especially on the coast and the plain. In the south-eastern regions (Ardennes), which are located at higher altitudes as well as at a greater distance from the sea, the climate is more continental, and also more unstable in summer.
Precipitation is frequent, but not particularly abundant. The coastal area is less rainy, and it is also slightly sunnier than the interior: precipitation amounts to 820 millimeters (32 inches) per year in Ostend, located on the coast, to 850 mm (33.5 in) in Brussels, a hundred kilometers (60 miles) away from the coast, and to 1,100 mm (43 in) in Spa, in the Ardennes.