Hah! I've been to Oulu. These people are hardy. Riding bikes in winter near the Arctic Circle is tough work. Yet the bike racks were full in January when I visited a few years ago.

It's not tougher than dressing up for going outside.

Heat and rain are bigger barriers to bicycles than cold. You could make a case for ice, but you can address that at the infrastructure level. You can make a case for hills, but again, infrastructure (namely, minimizing grade and going around whenever possible). It's a lot easier to wear a coat than it is to wear an air conditioner; if Tw > 30, you are risking heat stroke by pedaling. California, except the far southeast, has generally cooler and much drier summers than most of the United States.

Also, it's +2 in Umeå right now, which is warmer than I thought.

Heat can be mitigated too, to a certain extent. Shade from trees, reducing the urban heat island effect, and infrastructure to minimizing stop-and-go.

The biggest barrier to bikes is deadly cars.