Swell Train

Last few days, we encountered a swell train with a long period. It started two days ago and is diminishing today. Wavelengths and periods are the nasty parameters for the Thialf as soon as the period of the swell starts rising above 10 seconds, In short: wavelenghts of swell matching the length over all of the Thialf is not good. She'll starts rolling and pitching then sometimes beyond her operational limits. For us ? The longer the better, longer period swell stands for a good sleep, we are more influenced by short period swell. It is pretty dark on this picture, but the funny thing is that you can see very clearly the crests of the long period low swell.