[HPsquared]

The sun is at a lower angle from the horizon at higher latitudes. It passes through more atmosphere, losing some UV, and also the surface irradiance on the ground (energy per unit land area) is lower.

[flossly]

This. What I've learned for VitD conversion in the skin the sun needs to be more than 45° "high". Below that it's not much effective.

Those long summer evenings in northern Europe are thus not all that useful for VitD.