A controller for determining the presence of ice at a wind farm including a plurality of wind turbines (12a-e) each having a wind speed sensor (14a-e), the controller comprising an input and a processor: wherein, during a training period of the wind farm: the input is configured to receive: a reference ambient temperature of the wind farm; a measured reference wind speed from each of the wind speed sensors (14a-e); an estimated reference wind speed of each of the wind turbines (12a-e), wherein each of the estimated reference wind speeds is based on the measured power output of the associated wind turbine (12a-e); and, during the training period of the wind farm, the processor is configured to: determine a reference wind speed difference between the measured reference wind speed and the estimated reference wind speed for each of the wind turbines (12a-e); determine the reference delta distribution based on the processor is configured to determine the reference delta distribution based on those of the reference wind speed differences associated with the reference ambient temperature being greater than a reference threshold temperature, and wherein the input is further configured to receive: a current ambient temperature of the wind farm; a measured current wind speed from each of the wind speed sensors (14a-e); an estimated current wind speed of each of the wind turbines (12a-e), wherein each of the estimated current wind speeds is based on at least one measured performance parameter of the associated wind turbine (12a-e); and the processor is further configured to: determine a current wind speed difference between the measured current wind speed and the estimated current wind speed for each of the wind turbines (12a-e); determine a current delta distribution based on the current wind speed differences; determine whether an ice event has occurred, the determination being in dependence on the current ambient temperature and in dependence on a comparison between the current delta distribution and the reference delta distribution; and, an output configured to provide an outcome of the ice event determination. the input is configured to receive: a reference ambient temperature of the wind farm; a measured reference wind speed from each of the wind speed sensors (14a-e); an estimated reference wind speed of each of the wind turbines (12a-e), wherein each of the estimated reference wind speeds is based on the measured power output of the associated wind turbine (12a-e); and, a reference ambient temperature of the wind farm; a measured reference wind speed from each of the wind speed sensors (14a-e); an estimated reference wind speed of each of the wind turbines (12a-e), wherein each of the estimated reference wind speeds is based on the measured power output of the associated wind turbine (12a-e); and, during the training period of the wind farm, the processor is configured to: determine a reference wind speed difference between the measured reference wind speed and the estimated reference wind speed for each of the wind turbines (12a-e); determine the reference delta distribution based on the processor is configured to determine the reference delta distribution based on those of the reference wind speed differences associated with the reference ambient temperature being greater than a reference threshold temperature, determine a reference wind speed difference between the measured reference wind speed and the estimated reference wind speed for each of the wind turbines (12a-e); determine the reference delta distribution based on the processor is configured to determine the reference delta distribution based on those of the reference wind speed differences associated with the reference ambient temperature being greater than a reference threshold temperature, and wherein the input is further configured to receive: a current ambient temperature of the wind farm; a measured current wind speed from each of the wind speed sensors (14a-e); an estimated current wind speed of each of the wind turbines (12a-e), wherein each of the estimated current wind speeds is based on at least one measured performance parameter of the associated wind turbine (12a-e); a current ambient temperature of the wind farm; a measured current wind speed from each of the wind speed sensors (14a-e); an estimated current wind speed of each of the wind turbines (12a-e), wherein each of the estimated current wind speeds is based on at least one measured performance parameter of the associated wind turbine (12a-e); and the processor is further configured to: determine a current wind speed difference between the measured current wind speed and the estimated current wind speed for each of the wind turbines (12a-e); determine a current delta distribution based on the current wind speed differences; determine whether an ice event has occurred, the determination being in dependence on the current ambient temperature and in dependence on a comparison between the current delta distribution and the reference delta distribution; and, determine a current wind speed difference between the measured current wind speed and the estimated current wind speed for each of the wind turbines (12a-e); determine a current delta distribution based on the current wind speed differences; determine whether an ice event has occurred, the determination being in dependence on the current ambient temperature and in dependence on a comparison between the current delta distribution and the reference delta distribution; and, an output configured to provide an outcome of the ice event determination.