The Glossary of Meteorology defines a landspout as a "Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity [or rotation] originating in the boundary layer. The parent cloud does not contain a preexisting mid-level mesocyclone. The landspout was so named because it looks like 'a weak Florida Keys waterspout over land'".
Landspouts are a type of tornado that forms during the growth stage of a cumulus congestus cloud by stretching boundary layer vorticity (spinning) upward and into the cumulus congestus's updraft. This vertical stretching decreases the radius of the vortex and through the conservation of angular momentum, increases the spin rate. These generally are smaller and weaker than supercell tornadoes and do not form from a mesocyclone or pre-existing rotation in the cloud. Because of this, landspouts are rarely detected by Doppler weather radar.
Landspouts share a strong resemblance and development process to that of waterspouts, usually taking the form of a translucent and highly laminar helical tube. Landspouts are considered tornadoes because a rapidly rotating column of air is in contact with both the surface and a cumuliform cloud. Not all landspouts are visible, and many are first sighted as debris swirling at the surface before eventually filling in with condensation and dust.
Tornadoes can be classified into three groups depending on their formation. Type I is associated with a single, supercell thunderstorm. Type II occurs with quasi-linear convective systems such as squall lines. Type III forms when localized weaker convective cells interact with shear vortices in the boundary layer. These include cold-air funnels or snowspouts, waterspouts, and landspouts.
The convergence of surface boundaries with horizontal shear and thunderstorm updraft forms the circulation. The circulation is then then pulled in an upward direction to the base of the thunderstorm. A typical tornado/funnel cloud always forms from top to down, but the landspout always forms bottom up. A landspout generally lasts for less than 15 minutes; however, they can persist substantially longer, and produce heavy damage. They progress through recognizable stages of formation, maturation and dissipation, and tend to decay when a downdraft or significant precipitation occur nearby. They may form in lines or groups of multiple landspouts.
Landspouts are commonly weak; however, in rare occasions, a landspout can be as strong as an EF3 tornado.