Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. Weather scientists haven't found it so simple in practice, however, to classify and define tornadoes (per this essay by Doswell).
Bikini Island, 2015. Credit: Emlyn Hughes. (Phys.org)âA team of researchers from Columbia University in New York has found that all of the Marshall Islands involved in nuclear tests by the U.S. are now habitable, except for Bikini Atoll.
It could be that our home galaxy, the Milky Way, is not an entirely hospitable galaxy for life, according to a scientific report last month that suggests other galaxies in the universe could host 10,000x more habitable planets than the Milky Way. Read more about the most habitable galaxies in the universe here.