I am a bit new to compensations, but after spending 9 hours between flights, and then 4 more hours due to delay, I am wondering about my situation. When aircraft finally arrived to HEL, airline told us that delay was caused by weather conditions in AMS, and scheduled aircraft was not able to leave airport. Thus, airline assembled another crew and aircraft, which arrived to take us to our destination. Taking into account that weather conditions is something that airline cannot control, I wonder it is important that it was weather in third airport, not in HEL or PRG.
So in order to get out of paying compensation, the cause does not only have to be outside the airline's control -- it also has to be "extraordinary circumstances".
The European Court of Justice has taken a quite limited view of what counts as "extraordinary". Major (Eyafjallajökull-scale) disruptions probably will be, but ordinary weather conditions that don't make the news (except very locally where they occur) probably won't. Weather trouble is viewed as an ordinary risk of doing business for airlines.
Thus, go ahead and claim compensation. If the airline refuses, you will then have to decide whether to take them to court to enforce your rights. There are several specialized legal agencies that will generally take these cases on a "you win or it's free" basis, in exchange for a cut of the compensation you get.