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.
Bikini Atoll nuclear test: 60 years later and islands still unliveable. Marshall Islanders unable or unwilling to return to traditional home, scene of huge US hydrogen bomb test in 1954. Mushroom cloud from the Operation Castle Bravo nuclear explosion in the Bikini Atoll, Marshall Islands. Photograph: US air force.