It's rather impressive how much detailed weather information you can find about an area isn't it?
Yup, I do a lot of my planning for a trip based on that Weatherspark website for clothing planning, how much time I might choose to stay in one spot due to rain for distance planning, which sleeping bag I might want to bring, etc.
That site told me that for my Iceland trip, it would never get dark. I brought a bike light for tunnels, never used it. Brought a tiny little flashlight in case I might want to look for something in the tent at 2am, but only actually used the flashlight some times in the middle of the night in a hostel.
That site also told me that days would be short on my Florida trip in February, brought my best headlamp (for my head, not bike) on that trip.
That is one of the best sites I know of for generic trip planning across the globe.