At any rate another factor is that in the Northern regions it's my experience people do their best to soak up as much sun as possible.
I grew up in central Michigan and thought the same about the sun until I spent a few years in southern Arizona. The sun there feels like a heat lamp in summer. The low humidity makes the atmosphere absorb less energy. Although it feels like it's about 5 C cooler than it actually is because sweat evaporates so quickly, but once it gets to 43 C you actually just feel vaguely ill just going outside. Standing in the sun is just not something you do. It's a very unique experience to lick your lips and have your tongue feel cold.
Of course, neither of those were as bad as Houston in August. It was 38 C and 98 % relative humidity. And, I will point out, it had not rained. That's just what it's like. You walk outside and your glasses instantly fog up and you feel like you desperately need to shower.
Those long summer evenings in northern Europe are thus not all that useful for VitD.
Lower wavelengths like IR (heat) can more easily pass through the atmosphere. And since the Sun is lower in the sky, it hits more of your body. So you might feel more of the heat, while still getting less UV.