If a flight was cancelled due to "severe weather conditions, more specifically heavy snow, rain and fog, which made the operation of the flight impossible"
If the weather is that poor then no compensation is payable surely, how can the airline be responsible for the weather?
Just because other airlines were able to continue to operate means nothing unless they were operating the same aeroplanes, and even then there are differences. Your airline might operate aeroplanes that can't land in the thick fog that another airlines can, even with the same aeroplane type different companies have different limits on all sorts of things. In your example you quote "heavy snow", some operators might restrict operations on runways with different slippery conditions dependent on their risk assessment.