The weather in Kiel is always nice, you just need to have the right clothing. This summer seems to be on the cold/wet side, temperatures go up to about 20°C and down to 5-10°C at night. It is raining frequently (but in August/September the rain is warm!) and can be windy, so you should bring a sweater and a water/windproof jacket, especially when you go on the ferry and want to enjoy the trip on deck (you can go inside, but you want to be outside when you go with the ferry). You should NOT bring an umbrella, they usually do not last very long when it is windy and they are too dangerous when you are around other people. Only tourists carry an umbrella, for 10 minutes, then it goes into the trashcan.