Abstract: An apparatus providing for simultaneous measurement of the wind upstream and downstream of a wind turbine uses either a single LIDAR beam split into two beams, each focused upstream or downstream of the windmill, or a multiple beam LIDAR with a first beam source aimed toward the upstream direction of the wind and a second beam source aimed at the downstream direction after the wind has passed through the wind turbine. The apparatus may also use LIDAR to measure wind direction and speed by making measurements along slightly different lines of sight, or by pointing the LIDAR in different directions. Two lines of sight allow measuring wind direction in the plane defined by the two lines of sight. Three non-coplanar lines of sight provide the information necessary to determine a full 3-dimensional wind velocity vector. Further, LIDAR may also be used to measure wind speed by estimating the wind velocity using inputs from both aerosol and molecular components.