The Morning Glory
The Morning Glory is a spectacular roll cloud that frequently sweeps in low over Australia's Gulf of Carpenteria, often around sunrise in clear, calm weather. The cloud is only 100-200 m thick but very long and straight, extending from one horizon to the other. (One pilot followed if for 120 km without finding its end.) Sometimes as low as 50 m, the Morning Glory brings squall-like winds but rarely more than a fine mist. Double Morning Glories are not uncommon. Sev-en were once reported. Oriented NNW to SSE in the main, they advance east-towest low and fast (30-50 mph). Convincing explanations are wanting. One meteorologist has proposed that the Morning Glory is a "propagating undular hydraulic jump."
(Neal, A.B., et al; "The Morning Glory," Weather, 32:176, 1977.)