So, we have already considered Umester clouds and the Kelvin-Helmholtz instability.
Well, here's another interesting option.
Morning Glory cloud is a rare meteorological phenomenon, a type of cloud observed annually in the spring in the Gulf of Carpentaria in northern Australia and less often elsewhere. Clouds are a thunderstorm collar that can reach 1000 km in length, 2 km in height and travel at speeds up to 60 km / h.
Morning gloria is often accompanied by squalls, wind shear, and surface pressure surges. Ahead of the cloud, there is a rapid vertical movement that moves the air upward and "swirls" the cloud, while in the middle and back of the cloud, the air sinks down. In this case, a dangerous air situation arises for flying aircraft.
The nature of this unique phenomenon is not fully understood, and there are several hypotheses. Someone says that these are dormant tornadoes, someone - that these are spiral arms of cyclones, someone - that they resemble a squall gate, which sometimes appears just before the storm itself.
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The best place to watch these clouds is around Burktown, Australia, and the best time is around September and October (Australian spring).