If Aberdeen could rely on having great weather, it would be a world famous resort. It has amazing coastlines and beautiful countryside around it. Most people do not come to Aberdeen or the Grampian area for the weather. But it can't be all that bad as the Queen has her holiday home nearby and returns year after year. Donald Trump is about to build a golf resort to the north of Aberdeen.
June to September is the best time to visit if you are looking for warmer weather. Because Aberdeen is on the coast expect some wind off the sea to keep it cooler. Weather can be very mixed even in one day and inland will vary from the coastal loactions. Due to this tourist attractions tend to take this into account where possible and there are many more indoor than outdoor.
Here is the link to a weather site - www.wunderground.com/ where you can take a look at the seasonal averages.