Good question - In order to circle, you not only have to be able to see the runway but your weather has to be at or above circling minima. Typical visibility will be 2,400 metres and cloudabase must be at or above circling height (altitude) which varies from airport to airport. At Oslo it is 1,300', at Amsterdam and Inverness 880'. And circling may not be permitted at every airport, some due to terrain and at others "just because".