A capital in the Senne valley
Brussels is small on the map, yet the weather of the capital region is anything but uniform. The centre of Brussels lies at 60 metres, but municipalities differ by nearly seventy: Molenbeek-Saint-Jean in the Senne valley sits at 38, while Uccle in the south stands at 100. The Senne, now almost entirely vaulted beneath the city, formed the natural hollow, and slopes to the east and south rise gently from it.
The effects are small but real. In January the average night-time low in Molenbeek is 1.7 degrees, in Uccle 1.3. In July the city centre reaches 23.7, while Uccle stays cooler at 23.3. A tenth of a degree sounds like nothing, yet on clear winter nights it can decide whether roads are wet or frozen. City residents notice it less than the gardeners on the fringe.
Stone, concrete and the heat island
Dense building, little wind and extensive paving make Brussels a pronounced heat island. On hot summer nights brick gives back stored heat for hours, and the centre stays noticeably warmer than its surroundings. The capital collects about 865 millimetres of rain on 152 days a year, and it is mostly short, heavy late-summer showers that flood streets when drains and the Senne are overwhelmed.
Districts such as Schaerbeek, Anderlecht and Ixelles lie close together, yet thunderstorms often cross only part of the region. It happens that Anderlecht is under a black sky while Forest still has sunshine. On summer evenings it pays to check the radar before heading to an open-air concert.
Forest and landmarks
On the south-eastern edge begins the Sonian Forest, a beech wood about 120 metres up that runs beyond the city limits into Flanders and Wallonia. Beneath the canopy summer is distinctly cooler than in the streets, and the air stays moist. In autumn mist drifts between the trunks, and early risers in Watermael-Boitsfort enjoy still mornings with hoar frost on the clearings.
In the north the Atomium rises on the Heysel plateau, 56 metres above the sea. Visitors to the spheres see far across the city in clear weather, while under low cloud the view ends at the windows. Wind is stronger up here than in the centre, and in winter snow can linger longer on the spheres. Jette and Woluwe-Saint-Lambert lie at similar heights and share this weather style.
Winter weather and traffic
Unlike the Ardennes, snow in Brussels is rare, but the consequences are large. A few centimetres are enough to halt traffic on the ring road and in the tunnels, and freezing damp is treacherous on bridges. January is not cold, with a daytime mean of 6.4 degrees, but nights near zero are enough for black ice. Brief February cold snaps bring easterly wind and frost, and the map usually flags them two days ahead.
Fog is the second theme, especially in autumn high-pressure spells. At Zaventem airport it can disrupt flights; in town it lengthens the morning drive. Commuters check the 14-day forecast for Brussels and heed warnings from the weather institute. The institute itself is based at the Uccle observatory and has long supplied the country's reference measurements.
Neighbours and daily life
Around the capital lies Flemish Brabant with the Pajottenland and the Hageland, and to the south Walloon Brabant with the Waterloo plain. Crossing the city limit you hardly notice a difference in climate, though exposure to wind changes. On the map of Belgium you can follow how fronts from the west sweep the region.
For allergy sufferers the city is no refuge: plane trees, birches and limes flower along the avenues, and in spring pollen hangs in dense clouds. Our guide Pollen in Flanders and Wallonia classifies season and burden. And anyone unwilling to leave a bike behind in the rain at least has short distances in Brussels. Anyone leaving town for the weekend does best to compare Brussels with the surrounding country: in an easterly it is often cooler on the heights of Uccle, while in a south-westerly the warmth of the centre is an advantage.
Sources and further reading
IRM-KMI: Royal Meteorological Institute of Belgium
Waterinfo: river levels and flood situation in Flanders