I've spent a lot of time in Melbourne, and it's winters are characterised by almost constant heavy cloud cover. That mixed with greater latitude makes natural

I've spent a lot of time in Melbourne, and it's winters are characterised by almost constant heavy cloud cover. That mixed with greater latitude makes natural vitamin synthesis by UV a real problem. There is also a large hole in the ozone around the southern hemisphere, so maybe that allows a little more UV through on less cloudy days.