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Primary reference(s)

WMO, 2017. . Accessed 25 November 2019.

Additional scientific description

A large tornado in which the condensation funnel is at least as wide horizontally at the ground as it is in height from the ground to the cloud base may be referred to as a wedge tornado. During the dissipation stage of a tornado, the condensation funnel will shrink and narrow in width, becoming rope-like (a rope funnel), and may also become contorted. Some tornadoes may contain secondary vortices within the main circulation (suction vortices or subvortices) (WMO, 2017a).

Metrics and numeric limits

Tornadoes can be classified into the following distinct formation groups (WMO, 2017b): Type I (in association with supercells; WMO, no date), Type II (in association with quasi-linear convective systems), and Type III (localised convective and shear vortices – these comprise landspouts, waterspouts and cold-air funnels).

Definitions for the Type III tornados are as follows (WMO, 2017b):

• Landspout: A tornado that does not arise from organised storm-scale rotation and is therefore not associated with a wall cloud (murus) or a mesocyclone.
• Waterspout: A tornado occurring over water. It is normally a relatively small, weak rotating column of air over open water below a Cumulonimbus or Cumulus congestus cloud.
• Cold-air funnel: A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold.

The strength of a tornado can be estimated from the degree of damage caused using the Enhanced Fujita scale (Wind Science and Engineering Center, 2004; National Weather Service, no date).

Examples of drivers, outcomes and risk management

Owing to the unpredictable nature of tornados, protecting the public is focused on education and outreach which provide information on the tornado as a threat, how to identify a tornado and practical measures on how individuals can protect themselves, and how to find and watch warning systems that alert the public (CDC, 2020).

Since the advent of Doppler Radar, lead times for tornado warnings have increased from when a tornado first touches the ground to upwards of 14 to 20 minutes or more beforehand (WMO, 2017b; National Geographic, 2019).