People visit Oslo throughout the year in order to see the sights and soak up the atmosphere, and there is no specific high season for visiting. The climate is similar to that of Edinburgh, with cold winters, and warm summers, when the temperature can climb up above 20C. During spring and autumn you still need to wear a couple of layers to keep warm.
Although cold, many people prefer to visit Oslo during the winter months, because the frozen city becomes very picturesque, and there are plenty of places to go skiing nearby.
