Wave-like deformation of the line of a front between two air masses. The wave develops from the northward incursion of warm air and usually travels along the front, with colder air ahead and to the rear. Typically, frontal waves occur in sequences, or ‘families’, of several waves, and develop into depressions or storm centres travelling more or less eastward as ‘secondaries’ along the extended cold front to the rear of the original low. The secondaries tend to catch up and merge with the original depression as it slows up in its fully developed stage.