Severe Storm Definitions & Whirlwind Classification Scheme
Severe thunderstorm & severe local storm
At the fifth TORRO conference, held jointly with the Royal Meteorological Society at Imperial
College, London, in October 1994, David Reynolds presented TORRO's definition of a severe
thunderstorm and severe local storm.
TORRO defines a severe thunderstorm as a thunderstorm which has:
- one or more tornadoes and / or one or more waterspouts,
- hail intensity of at least H3 at ground level,
- non-tornadic winds gusting to 100 km h-1 or more at 10 m above the ground.
As these elements may also occur without thunder (although in reality, a H3 hailstorm is highly
unlikely to be thunderless) so a wider definiton of a severe local storm can then be made.
Thus, TORRO defines a severe local storm as:
- 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:
(i) one or more tornadoes and / or one or more waterspouts,
(ii) hail intensity of at least H3 at ground-level,
(iii) non-tornadic winds gusting to at least 100 km h-1 at 10 m above the ground (but which are
not part of synoptic-scale straight-line winds of such velocities).
The additional statement at the end of (iii) is required in order to exclude the widespread high winds
brought by deep depressions.
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 are 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). There is no nomenclature sub-division of tornadoes, waterspouts and funnel clouds, although
they can all 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 are (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
An abbreviation in upper-case indicates certainty in the report, while the use of lower-case indicates
events where a small degree of uncertainty exists about the true cause. For example, TN indicates a
definite tornado, while tn indicates a probable tornado.
However, in all instances the Beaufort letters q (for squall) and kq (for line squall) remain lower case
regardless of their certainty.
It should be pointed out that ball lightning is not accepted by some scientists, while others are
uncertain of its existence. From that stand-point, the use of the abbreviations BL and bl in this context
would not be acceptible. It is the opinion of TORRO, as well as of many other scientists, that - in the
balance of probability - ball lightning does exist as a real phenomena, whose nature is currently
uncertain.
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