circumstances under its control
This is the classic air industry argument.
Easyjet will argue that your evening flight being cancelled because some delay in Spain or wherever caused their morning flight to run late and hence the rest of the day ran late and now the staff don't have sufficient working hours to fly your flight is due to "circumstances outside of its control", when clearly it could have been avoided had they been better prepared and/or scheduled a bit more slack into the system to cope with such eventualities.
Given that, I'm sure BAA can argue that any sort of weather-related incident is outside of its control.