However, in North America, tornadoes occur with a relatively high frequency, as the conditions there in spring and summer are often just right for supercell storms to develop. Down the middle of North America runs a strip of land, roughly from Arkansas to Colorado and all the way from Texas to North Dakota, which is called Tornado Alley, due to the high number of tornadoes striking there. As said, a vortex or so-called funnel cloud is a tornado as soon as it reaches the surface. However, many more vortices never make to the ground. The funnel clouds occasionally seen under big convective clouds, other than supercell storms, are called landspouts.
Waterspouts are tornadoes over water.
These need not be as big, and are not necessarily spawned by supercells. They frequently occur over warm water surfaces under convective clouds. There must not be too much wind, and there must be sufficient instability and moisture, for a waterspout to form. When a waterspout reaches land, it becomes, technically speaking, a tornado, but quickly dissipates. Another type of vortex in the atmosphere is the dust devil.