Map from Google - Cologne

In Cologne, a city located in Western Germany, in the state of North Rhine-Westphalia, as well as in nearby Leverkusen, the climate is sub-oceanic, humid and rainy, influenced by the Atlantic Ocean, so it has cold (but not freezing) winters, and mild summers.
Cologne is is the largest city of the metropolitan region called Rhine-Ruhr, which includes in addition to those of the Ruhr, cities such as Düsseldorf, Wuppertal, Leverkusen, Mönchengladbach and Bonn.
The average temperature in January is 2,5 °C, while in July it's 19 °C.
Here are the average temperatures.
Average temperatures - Cologne
CologneJanFebMarAprMayJunJulAugSepOctNovDec
CologneJFMAMJJASOND
Min (°C)-1-124811131310730
Max (°C)571115192224242015106

Precipitation is relatively abundant, about 800 millimetres per year, but above all it is common and 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 shower or drizzle, whereas during the summer, thunderstorms can break out in the afternoon.
Here is the average precipitation.
Average precipitation - Cologne
CologneJanFebMarAprMayJunJulAugSepOctNovDecYear
CologneJFMAMJJASONDY
Prec. (mm)604565507090856570606570795
Days1291310101211911111213133

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

Cologne

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 Eastern Europe, the temperature drops below freezing, for longer or shorter periods, even during the day. The temperature rarely drops below -15 °C, however, it can do it in very cold periods, which in the past decades were more frequent. Here are some values recorded in the past: January 1940: -19 °C; January 1942: -25 °C; January 1979: -20 °C; January 1985: -23 °C; February 1986: -17 °C; January 1987: -17 °C; February 1991: -17 °C.
Snowfalls in Cologne 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.
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 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, 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 30/32 °C. The highest record is 39 °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 Cologne 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 Cologne 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.