Wind is the large-scale movement of air from one place to another. It is usual for the air to become horizontally stratified, the layers nearer the ground moving more slowly than or in a different direction to those higher up. This change of wind speed with height is known as wind gradient (or wind shear), and it may cause serious problems for aircraft during their approach and landing. Unfortunately, at present there is no satisfactory way of ascertaining the wind gradient situation. The invention provides acceptable apparatus for, and a method of, actually detecting--and preferably measuring--wind gradient at a location, and involves a comparison of the wind speed in the same direction at two or more heights at the location, this comparision being upon the basis of a comparison of the speed of sound in that direction and at those heights, a difference in the apparent speeds indicating the presence of wind gradient. The invention involves: beaming a regular sound wave train between a transmitter/receiver pair positioned and like orientated at each of two or more heights at the location; noting each transceiver pair's received sound wave train phase, and comparing it with its transmitted phase, so as to deduce the wind-caused phase change; and using these deduced phase changes to calculate the actual wind speeds, and thus the relative changes of wind speed with height, in the selected direction.