A method and apparatus for determining the velocity of wind (or another fluid) in which one or more receiving acoustic detectors at (e.g. a location (A(r,t))) on the ground are arranged to measure the phase direction of arriving waves from one or more sound sources (at locations S(r,t)), all of which may be fixed or moving. A mean value of the wind and its gradient can be obtained along the sound rays (R) between the sources and the detectors. Typical sources may be aircraft or other flying vehicles or objects, or sound sources arranged particularly for the wind measurement. For example at airports, where conventionally the positions of the air vehicles are often rather accurately known, several sets of detectors can be provided for monitoring the wind conditions and for issuing alarms in the case where extreme winds or wind changes are detected.