Severe Storm Definitions & Whirlwind Classification

Severe Thunderstorm & Severe Local Storm
TORRO defines a severe thunderstorm as a thunderstorm which has one or more of the following:

  • one or more tornadoes and / or one or more waterspouts,
  • hail intensity of at least H3 at ground level,
  • non-tornadic winds gusting to 55 mph or more at surface.
  • a thunderstorm which fulfils the criteria of a severe thunderstorm and, additionally,
  • a thunderless storm which is accompanied by one or more of the following:
    • one or more tornadoes and / or one or more waterspouts,
    • hail intensity of at least H3 at ground-level,
    • non-tornadic winds gusting to at least 55 mph at surface (but which are not part of synoptic-scale straight-line winds of such velocities).

Whirlwind Classification Scheme
Dr. Terence Meaden devised TORRO's whirlwind classification scheme, which is outlined below.

Vortex, whirlpool and whirlwind
As a preamble, a vortex may be defined as a rotation within a fluid. If the fluid is a liquid, the vortex may be more accurately termed as a whirlpool, while if it is a gas it is termed a whirlwind.
Major, minor and other whirlwinds
TORRO then sub-divides a whirlwind into major, minor and other whirlwinds
Major whirlwinds: funnel clouds, tornadoes and waterspouts
A major whirlwind may be defined as a helical rotation which is joined to a cumulonimbus cloud, or a towering or fast-growing cumulus cloud - and hence is associated with unsettled weather. The rotation will decend from the cloud-base as it develops and then retract back into the cloud-base as it decays. If the concentrated rotation does not reach the surface, the major whirlwind is termed a funnel cloud. However if the concentrated rotation reaches land the major whirlwind is termed a tornado, while if it reaches a water body (such as the sea, a lake or reservoir) it is termed a waterspout. A tornado may become a waterspout as the rotation moves from land to sea (and vice-versa).  Such whirlwind can develop in a number of differing synoptic and mesoscale conditions.
Minor whirlwinds: land devils, water devils and fire devils
A minor whirlwind may be defined as a helical rotation which develops at the surface and rises into the sky, usually (but not always) as the result of the strong heating of that surface by the sun in calm or light winds - and hence is (usually) associated with fair or fine weather. If the rotation develops over land, the minor whirlwind is termed a land devil, while if it develops over water it is termed a water devil. Intense devils may eventually be topped by a small cumulus. Note that minor whirlwinds should not be classified according to what material they are raising, as this would invoke a whole host of names (such as dust-, ash-, straw-, hay-, litter-, leaf-, snow- and even "invisible" devils) all for the same phenomena. A whirlwind may also develop over a fire or very hot ground, if there is a suitable convergence of - and rotation in - the air. This, the fire devil, is also classed as a minor whirlwind as it develops upwards from the surface. As the heat source produces instability in the air, fire devils occur regardless to the prevailing weather conditions - although, as with other minor whirlwinds, there is a greater incidence of formation in calm or light winds as this allows heat to build up more quickly. Fire devils most commonly occur during forest fires, wild fires and stubble burning, but have also been observed with other fires such as bonfires, oil fires, volcanic eruptions and nuclear explosions.
Eddy whirlwinds
One species of whirlwind called the eddy whirlwind can be considered both a major and a minor whirlwind, depending on the prevailing conditions. The eddy whirlwind usually forms in the lee of an obstacle, but may also form at the interface of two masses of air which are moving at different speeds. They may occur on any scale from the lee of a mountain to near to a wall in a street. Whirlwinds previously considered are driven by the movement of warmer air into cooler air, but with an eddy whirlwind there is little heat movement - the eddy's driving source is mostly momentum and consequently are more likely to occur in breezy or windy conditions.
Other whirlwinds
The third grouping, other whirlwinds, includes those such as trailing vortices. Trailing vortices develop from aircraft wings and are most commonly seen when the air is moist. They develop as a consequence of a difference in pressure above and below the wings and also the aircraft's forward movement. Trailing vortices may also develop from suitably-shaped hills (usually conical in shape or with another form of sharp break in slope).

Abbreviations
The following abbreviations are regularly used by TORRO, especially in reports and databases:

  • BL - ball lightning
  • EF - St. Elmo's Fire
  • EW - eddy whirlwind
  • FC - funnel cloud
  • FD - fire devil
  • HS - hailstorm
  • LD - land devil
  • RS - remarkable shower
  • TN - tornado
  • WD - water devil
  • WS - waterspout
  • WW - whirlwind

Upper-case indicates definite, and lower-case indicates probable.