Found a really useful site to get historical average weather conditions for a given month -- weatherspark.com. It covers every single place I'll be visiting on my Iceland & Greenland cruise, including the Prinz Christian Sund waterway that we'll be traveling through when we leave Qaqortoq.
You can view the average data for an entire month, and it gives a ton of useful information -- high-low temperature range, hours of daylight (useful in the far north!), average rainfall chance and total over the month, average wind speed and direction (hugely useful in a place like Prinz Christian Sund, where we're standing on deck much of the day observing the scenery and need to dress warmly enough), average water temp (I remember @twangster's comment about how, absent any major land nearby, ambient air temp is driven by the water temp), and more.
This just made planning our wardrobe and what layers to pack a lot easier! Figured I'd share since I know there are others here who've been trying to figure out what to pack for a cruise that changes climates, and visits variable climates, kind of quickly over a few days.