2. Waterspouts, landspouts, and ...?
Having mentioned waterspouts, this raises another topic. There is a special name for a tornado moving over the water: a waterspout. Why do we not have special names for tornadoes moving over sand (sandspout?), or asphalt (tarmacnado?), or mobile homes (manufacturnado?), or eucalyptus trees (gumswirl?)? Is it a waterspout if the water is fresh water rather than sea water? Does it become a waterspout if it moves over a lake? What about a pond? How about encountering a swimming pool or perhaps a puddle? How big does a body of water have to be to create a waterspout from a tornado? What about when crossing a river? A creek? A dry streambed? Would this last example be a "dry waterspout"? I am engaging deliberately in reductio ad adsurdum here because I do not believe there is any scientific distinction of consequence between a waterspout and a tornado!
In the new Glossary, in fact, the definition of a waterspout is now:
Waterspout -- 1. In general, a tornado over a body of water. 2. In its most common form, a nonsupercell tornado over water.
For years, people believed that waterspouts were a distinctly different phenomenon, uniquely associated with tropical and subtropical convection that might not even qualify as cumulonimbus clouds. Of course, some "authorities" knew of the annoying problem of supercells over water; recognition of this produced the abominable term: "tornadic waterspout." Of late, it has been observed that phenomena quite comparable to waterspouts arise over the land, leading to another dubious term (that I have used!): "landspout" (by analogy, a "waterspoutic tornado"?). In my opinion, all these terms refer to the same phenomenon: an intense vortex associated with deep moist convection. Thus, I must quibble with the standard definition for its exclusion of convective vortices that happen with clouds not meeting the criteria to be cumulonimbi (e.g., those without glaciation at the cloud top).
I am proposing the following definition:
Tornado -- A vortex extending upward from the surface at least as far as cloud base (with that cloud base associated with deep moist convection), that is intense enough at the surface to do damage should be considered a tornado.
This is without regard to
* the underlying surface,
* the existence/non-existence of a condensation cloud from cloud base to the surface,
* the depth of the moist convective cloud,
* the presence/absence of ice in the upper reaches of the convective cloud,
* the occurrence/non-occurrence of lightning within the convective cloud, or even
* the intensity of the phenomenon beyond some lower threshold.
My broadened definition is designed to ignore what I consider to be incidental aspects of the situation. I believe that the physical process giving rise to an intense vortex is not associated with any of these coincidental issues and so the labeling of the real vortices that occur should not depend on them. It also excludes any phenomena not associated with deep moist convection, such as dust devils or "mountainadoes," and avoids making artificial and scientifically unjustified distinctions between "spouts" and tornadoes.[5]
I hasten to add that I do not believe that the physical processes giving rise to tornadoes are all the same. It appears that tornadoes arise in many different ways, and perhaps different process can be associated with the tornado at different times in its life cycle. Moreover, not all tornadoes associated with a given moist convective cloud arise via the same processes (see Doswell and Burgess 1993). Some of the relatively intense vortices associated with a convective storm probably should not be considered tornadoes; e.g., circulations not extending to the surface, and true gustnadoes (see below), assuming we can identify them as such. There is a fair amount of anecdotal evidence for non-tornadic intense vortices in association with convection (see Moller et al. 1974; Cooley 1978; Doswell 1985; Bluestein 1988; Doswell and Burgess 1993; Bluestein 1994), but not much hard information about the processes giving birth to these vortices.
At present, we are more or less content to classify tornadoes according to whether or not they occur with supercells. In the future, it may become scientifically useful to sub-classify tornadoes even further, as we learn more about how real events occur (as opposed to, say, events in our computer simulations!). If we must classify, then it seems to me that we should do so on the basis of physical processes and not be concerned with superficial aspects of the events. We are far enough along in our understanding of tornadoes that we ought to be able to move at levels deeper than the surface now.
