A monitoring apparatus for identifying a yaw misalignment of a wind turbine in a wind farm comprising a number of wind turbines (1-1 to 1-20), said monitoring apparatus comprises - an sampling device (7, 107) connected to the wind turbines (1-1 to 1-20) of the wind farm for retrieving at least the yaw angle of a wind turbine to be checked for a yaw misalignment and the yaw angles of a subset of wind turbines (1-1 to 1-20) of the wind farm, characterised by - an average value calculator (15, 115) connected to the sampling device (7, 107) for receiving the yaw angles of the subset of wind turbines, the average value calculator (15, 115) being adapted for calculating an average value of the received yaw angles; - a subtractor (17, 117) connected to the average value calculator (15, 115) for receiving the average value and connected to the sampling device (7, 107) for receiving the yaw angle of the wind turbine to be checked for yaw misalignment, the subtractor (17, 117) being adapted to calculate a difference value representing a difference between the yaw angle of the wind turbine to be checked and the average value; - a comparator (19, 119) connected to subtractor (17, 117) for receiving the difference value, the comparator (19, 119) being adapted to compare the difference value to a negative threshold and a positive threshold and to output the information signal if the negative threshold is undershot or the positive threshold is overshot, or to compare the absolute value of the difference value to a single positive threshold value and to output a information signal in case the threshold is exceeded; - a selector (11, 111) for selecting a wind turbine of the wind farm to be checked for yaw misalignment; - a rule storage (13, 113) with rules relating to generation of a subset of wind turbines (1-1 to 1-20) of the wind farm; and - a subset generator (9, 109) connected to the selector (11, 111) for receiving information about which wind turbine is selected and to the rule storage (13, 113) for receiving the rules for generating the subset, the subset generator (9, 109) being adapted to generate a subset of wind turbines based on which wind turbine is to be checked. - an sampling device (7, 107) connected to the wind turbines (1-1 to 1-20) of the wind farm for retrieving at least the yaw angle of a wind turbine to be checked for a yaw misalignment and the yaw angles of a subset of wind turbines (1-1 to 1-20) of the wind farm, characterised by - an average value calculator (15, 115) connected to the sampling device (7, 107) for receiving the yaw angles of the subset of wind turbines, the average value calculator (15, 115) being adapted for calculating an average value of the received yaw angles; - a subtractor (17, 117) connected to the average value calculator (15, 115) for receiving the average value and connected to the sampling device (7, 107) for receiving the yaw angle of the wind turbine to be checked for yaw misalignment, the subtractor (17, 117) being adapted to calculate a difference value representing a difference between the yaw angle of the wind turbine to be checked and the average value; - a comparator (19, 119) connected to subtractor (17, 117) for receiving the difference value, the comparator (19, 119) being adapted to compare the difference value to a negative threshold and a positive threshold and to output the information signal if the negative threshold is undershot or the positive threshold is overshot, or to compare the absolute value of the difference value to a single positive threshold value and to output a information signal in case the threshold is exceeded; - a selector (11, 111) for selecting a wind turbine of the wind farm to be checked for yaw misalignment; - a rule storage (13, 113) with rules relating to generation of a subset of wind turbines (1-1 to 1-20) of the wind farm; and - a subset generator (9, 109) connected to the selector (11, 111) for receiving information about which wind turbine is selected and to the rule storage (13, 113) for receiving the rules for generating the subset, the subset generator (9, 109) being adapted to generate a subset of wind turbines based on which wind turbine is to be checked.