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.
Photograph by Bill Curtsinger, National Geographic. A sea cucumber floats in the waters of Bikini Atoll, once home to a series of infamous nuclear bomb tests. These echinoderms can grow to 6.5 feet (two meters) by feeding on tiny aquatic animals, algae, and even waste material. Sea cucumbers recycle food particles into fodder for bacteria much like worms do in soil.