A propagating disturbance that is characterized by a sudden and relatively permanent change in the height of a horizontal fluid interface and in the velocity of the fluid beneath the interface with oscillations of its depth, and of wind and temperature characteristics within it. A classic example in the atmosphere is the morning glory phenomenon in Australia triggered by the sea breeze, which is characterized by a smooth band of cloud along the leading edge reminiscent of a gust-front arc cloud, and other interface bands along the wave crests of the following lower-amplitude waves. Undular bores can also occur in advance of cold-air outflows (gust fronts) from thunderstorms. Compare gravity current.