Norway is not a warm country. Arendal is in the south, and therefore generally has the best summer weather. Late July and early August is also the warmest time of year. That said, evenings can still get chilly this far north. It is highly recommended to bring some warm clothes, especially if you’re camping outside.
While the temperature may rise above 30 degrees (86 F) on the warmest days of the year and there are a handful of nights most summers when nighttime temperature does not drop below 20 degrees, (68 F) it will more likely peak at around 20-25, (70-80 F) and drop to 12-15 (53-59 F) at night.
In summary, bring sunscreen and a warm jacket.