The airline is not liable under EU regulation EC 261 and other similar air passenger laws if the flight disruption was due to “extraordinary circumstances.” These are scenarios which could not have been avoided even if all reasonable measures had been taken and include situations like medical emergencies, labor strikes, serious adverse weather conditions, and Air Traffic Control restrictions, to name a few.
In addition, events that qualify as “Force majeure,” such as wars and riots, as well as “Acts of God”—hurricanes, earthquakes, etc.—also exempt the airline from liability.
However, the airline still has an obligation to make sure you arrive to your final destination as soon as possible under similar air traffic conditions. This means that they have to arrange a new flight to you free of charge even if the flight was cancelled due to bad weather. The air carriers are also obliged to offer meals, refreshments, hotel accommodation, etc. if needed.