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.
The habitable zone defines the region where a planet might be able to retain liquid water on its surface. Any closer to the star and water would vaporize away; any farther, and it would freeze to ice. But water in its liquid state is what scientists are after, since that is thought to be a prerequisite for life.