What is a tornado and how is it related to a funnel cloud?

A tornado is a violently rotating column of air that extends through the lower part of a storm cloud to the surface (land or water). A rotating, typically funnel-shaped cloud often develops when lower pressure and temperature in the core of the tornado cause water vapour in the air to condense. This funnel cloud may extend fully or partly from the storm cloud to the surface with a tornado. Note that a waterspout is simply a tornado that occurs over water.

For NTP to verify a tornado from a report (photograph, video and/or detailed eyewitness description), there needs to be evidence of a vortex extending between a storm’s updraft and the surface - with surface winds associated with the vortex approaching or exceeding the lower bound of EF0 on the Enhanced Fujita Scale.

The report should therefore include a well-developed funnel cloud extending fully from the storm cloud to the surface, or a funnel cloud extending partly to the surface plus an area of rotating debris near the ground / 'spray vortex' near the water surface. Also acceptable is a narrow vortex over land made clearly visible by rotating debris and extending well above the surface under a storm’s updraft region. 

It is possible for a storm to generate a vortex that is too weak at the surface to be considered a tornado. Such a vortex may be accompanied by a funnel cloud aloft.

It is also possible for a short-lived, shallow vortex to develop along the leading edge of a storm downdraft (known as the ‘gust front’), well away from the storm updraft region. Such a vortex can generate an area of rotating debris near the ground or a 'spray vortex' over water, but is not considered a tornado. It is often referred to as a ‘gustnado’. Other vortices not considered to be tornadoes include dust devils and steam devils.