Map from Google - Dortmund

In Dortmund, a city located in the state of North Rhine-Westphalia, the climate is sub-oceanic, humid and rainy, influenced by the Atlantic Ocean, so it has cold (but not freezing) winters, and mild summers.
Dortmund is the largest city in the Ruhr area, a conurbation that includes several cities, among which Bochum, Bottrop, Dortmund, Duisburg, Essen, Gelsenkirchen, Hagen, Hamm, Herne, Mülheim an der Ruhr and Oberhausen.
The average temperature in January is 1,5 °C, while in July and August it's 17,5 °C (63.5 °F).
Here are the average temperatures.
Average temperatures - Dortmund
DortmundJanFebMarAprMayJunJulAugSepOctNovDec
DortmundJFMAMJJASOND
Min (°C)-1024811131311731
Max (°C)4591318212222191485
Min (°F)303236394652555552453734
Max (°F)394148556470727266574641

Precipitation is relatively abundant, about 800 millimetres (31.5 inches) per year, but above all it is common and well distributed over the seasons; however, the rainiest season (at least in quantity) is summer, the least rainy season is spring. Rainfall often occurs in the form of short showers or drizzle, whereas during the summer, thunderstorms can break out in the afternoon.
Here is the average precipitation.
Average precipitation - Dortmund
DortmundJanFebMarAprMayJunJulAugSepOctNovDecYear
DortmundJFMAMJJASONDY
Prec. (mm)655055606580857565557075795
Prec. (in)2.622.22.42.63.13.332.62.22.8331.3
Days1291310101211911111213133

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

Dortmund

Winter, from December to February, is cold: the daytime temperatures are on average a few degrees above freezing (0 °C or 32 °F), 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 (50/54 °F), but there may be wind and rain.
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 -15 °C (5 °F), however, it can do it in colder periods, which in the past were more frequent; here are some values recorded in the past: january 1940: -23 °C (-9 °F); january 1942: -28 °C (-18 °F); december 1996: -15 °C (5 °F); january 1997: -17 °C (1 °F); february 2012: -15 °C (5 °F).
Snowfalls in Dortmund 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 (50 °F), can occur also in May, especially in the first half of the month.
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 (77 °F) 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 quite often from the North Sea, which is not far, and even rainfall is quite frequent.
However, there are pleasant periods, in which the daytime temperatures are between 20 and 25 °C (68 and 77 °F), but also cooler periods, in which even the daytime temperatures remain below 20 °C (68 °F). At night it can be a bit cold, and the temperature can drop below 10 °C (50 °F) even in July and August. Sometimes there can be hot days, in which the temperature can reach 30/32 °C (86/90 °F), and more rarely it can exceed even 35 °C (95 °F) (in July 2006 and in 2015 the thermometer reached 36 °C or 97 °F).

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 Dortmund 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 6 hours of sunshine per day from May to August.

When to go

The best time to visit Dortmund 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.