PROJECT ASSESSMENT Delays caused by adverse weather are a major problem during offshore wind farm installation. The consequences are a greater demand for staff, vessels and materials, and thus higher expenses as well as losses due to later-than-planned commissioning. Most offshore activities cannot be executed in every kind of weather. For example, the sea state during foundation installation must not exceed a given threshold for a defined working period. These requirements, consisting of a minimum working period and one or more restrictions such as sea state, define weather windows. Weather windows are specified for the vessels, materials or procedures used. If prevailing weather conditions don’t match the required weather window, work has to be delayed. The following article by Gerrit Wolken-Möhlmann, research associate at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) Nordwest, Dr Thomas Bruns, head of the Hamburg branch of the German Meteorological Service (Deutscher Wetterdienst – DWD), and Carola Heitmann-Bacza, maritime meteorological consultant at the Hamburg Maritime Weather Office (Seewetteramt Hamburg), describes the methodology of defining favourable weather conditions.