The National Transportation Safety Board has released an advisory regarding Meteorological Evaluation Towers (METs). These towers are installed to gather wind data from areas that may be used for wind development projects. The towers are generally less than 200 feet tall and are typically not lit or marked well.
These factors can make them difficult to see from the air and can pose a danger to helicopters during the landing and takeoff portions of flights. The towers are often temporary and are erected overnight, making them hard to track.