So I will start with a first picture from Australia (credit Mick Petroff).
Explanation: What causes these long, strange clouds? No one is sure. A rare type of cloud known as a Morning Glory Cloud can stretch up to 1000 km long and occur up to 2 km high. Although similar roll clouds have been seen at specific places across the world, the ones over Burketown, Australia occur predicably every spring. Long, horizontal, circulating tubes of air might form when flowing, moist, cooling air encounters an inversion layer, an atmospheric layer where air temperature atypically increases with height. These tubes and surrounding air could cause dangerous tubulences for airplanes when clear.