Holland lies on the sea coast, so as a whole in the country a marine temperate climate with wet air and not a large fluctuation of temperatures.
Rotterdam is the second largest city of the Netherlands, located in the South Holland region, where the River Nivee Maas falls into the North Sea. The city stretched along the river and on the sea coast. Such a number of water causes a fairly unfortunate and mild climate. In principle, in Rotterdam is not very hot or very cold, but the wet air makes its business and low temperatures of about 0 or even just above are simply disgusting.
In the summer, the temperature in Rotterdam does not rise above +22, during the day is usually sunny and without precipitation. And in winter, the temperature is not lower below -1, but only such a small minus is quite unpleasant, the air becomes raw and divergent.
Rotterdam mainly go to see the city with its cubic houses and unusually beautiful landscapes, but also in the city there are beaches, a length of 9 km wide 100 meters. Only here the climate is not hot enough for a beach holiday, although someone and 25 cold, and someone and 20 are already 20.
So if you are traveling just to walk around the city and look at unusual at home, you can go in April-June, when the temperature ranges from 17 to 20 degrees, spring is usually solar here, so you can walk into your pleasure. If, in addition to Rotterdam, you want to visit Kekenkhov Park and the Tulip Festival on 32 hectares of the park, then it is worth going in the middle of March - late April, in the season of flowering of these beautiful plants, and then the temperature will keep approximately 10-17 degrees of heat.
If you still want to try to swim in the sea, it is worth going to the traditionally hottest time July-August. Only here the water "like pair milk" is not expelled, because water does not heat up to more than 17 degrees. Probably why Rotterdam is not considered a beach resort town.
In winter, the weather in Rotterdam does not have fun for walking and relaxing in the city. The air temperature ranges from -3 to +5, and from November to January there are rains with wet snow, or just very cloudy and foggy.
But on the other hand, nature has no bad weather, every weather is grace.! :) P.S. But for traveling still there is bad weather :)