Not just cloud cover. Most areas in the PNW, the sun is so low in the sky between October and March that you can't synthesize vitamin D through the skin at all during those months, even on a bright sunny day.
Even during the summer up here, you really only get a window of roughly 10am to 3pm where enough UV-B rays can penetrate the atmosphere, in July. It's estimated that >80% of the PNW population are deficient (compared with 40% nationwide in the US).
The body stores it though. So how much of a deficit you’re running for how long matters.
But yeah. Low vitamin D levels are common even with lily white people in Northern Europe, and at least here in Norway everyone with dark skin knows that they need vitamin D supplements. Traditionally, public health recommendation (for everyone) was to take cod liver oil regularly for every month with an R in it.
