McMurdo Station in Antartica (operated by the USA) is 77.85 degrees south (and 166.69 east), Longyearbyen (Norway) is the northernmost settlement with scheduled flights all year at 78.24 degrees north (and 15.47 east).
Longyearbyen is further north than McMurdo is south, but McMurdo rarely receives flights in winter, and seems to be a much more hostile environment overall.
Why is that?