Map from Google - Brussels

In Brussels, the capital city of Belgium, and home to important European institutions, the climate is sub-oceanic, humid and rainy, influenced by the Atlantic Ocean, so it has cold (but not freezing) winters and quite cool summers.
The average temperature in January is 3.5 °C, while in July it's 18.5 °C.
Here are the average temperatures.
Average temperatures - Brussels
BrusselsJanFebMarAprMayJunJulAugSepOctNovDec
BrusselsJFMAMJJASOND
Min (°C)1136912141411842
Max (°C)671014182123231915106

Precipitation is relatively abundant, about 820 millimetres per year, but above all it is common and distributed throughout the year; however the rainiest season (at least in quantity) is summer, the least rainy season is spring. Rainfall often occurs in the form of short shower or drizzle.
Here is the average precipitation.
Average precipitation - Brussels
BrusselsJanFebMarAprMayJunJulAugSepOctNovDecYear
BrusselsJFMAMJJASONDY
Prec. (mm)706070506575707565707080820
Days131113101110101010111213132

The wind blows frequently, and can be intense, especially from November to March.

Brussels

Winter, from December to February, is cold: the daytime temperatures are on average a few degrees above freezing, and at night they stay around freezing; wind and humidity often exacerbate the feeling of cold.
However, different weather situations may occur.
When the westerlies blow, the temperatures can be quite mild, and can reach 10/12 °C.
On the contrary, when the city is reached by cold air masses from Russia, the temperature drops below freezing, for longer or shorter periods, even during the day. The temperature rarely drops below -10 °C, however, it can do it in the coldest periods. Here are some values recorded in the coldest winters: January 1985: -16 °C; January 1987: -14 °C; December 1996: -12 °C; February 2012: -14 °C.
Snowfalls in Brussels are quite frequent, though usually not abundant, and occur in the form of sleet or light snow, sometimes however snow can accumulate on the ground, and last for many days, if it occurs during cold waves.

Spring, from March to May, is initially cold, with possible snowfalls still in March, and then it gradually becomes milder in April and May, although some cold days, with highs around 10 °C, can occur also in May, especially in the first half of the month. However, April and May are, albeit slightly, the least rainy and cloudy months.
Typically, in the second half of May the temperatures become permanently acceptable, and there can be the first pleasantly warm days, with highs around 25 °C or even above.

Summer, from June to August, is mild, and the flow of Atlantic fronts does not cease even in this season; the wind blows constantly from the sea, and even rainfall is quite frequent. However, there are pleasant periods, when the daytime temperatures are between 20 and 25 °C, but also cooler periods, in which even the daytime temperatures remain below 20 °C. At night it can be a little cold, and the temperature can drop below 10 °C even in July and August. Sometimes there can be hot days, in which the temperature can reach 28/30 °C, but this may not occur for a whole month. Here are some values reached in recent hot summers: August 2003: 35 °C; August 2013: 34 °C; July 2015: 34 °C.

Autumn, from September to November, is initially pleasant, ie in the first half of September, then it gradually becomes more cold, dull, wet and windy. The first night frosts usually occur in November, but sometimes also in October.

The amount of sunshine in Brussels is not high: the sun rarely shines from October to February, while it goes a bit better from May to August, when the weather is variable, and between a disturbance and the other the sun can come out. There are on average 7 hours of sunshine per day from May to August.

When to go

The best time to visit Brussels runs from mid-May to mid-September, being the mildest of the year. July and August are the warmest months; however, it's better to bring a sweatshirt or sweater, a light jacket and an umbrella, for cool and rainy days.