Page:Hunt - The climate and weather of Australia - 1913.djvu/167

Rh.

. 154 159. The name "tornado" is given to an excessively violent and destructive wind storm affecting only a narrow strip of country, and producing its destructive effects, not by a straight blow, but by air in rapid rotation, as in a whirlwind. From the latter, which is relatively insignificant, the tornado differs essentially, as it does also from the vastly wider cyclone, though the terms are often confused. The tornado is always associated with thunder and hail storms of extremely violent type. If we regard a thunderstorm as due to the uprushing of a column of air, from, say, the 4,000 to the 20,000 or even 30,000 feet level this giving rise to, and in turn being maintained by, the condensation of aqueous vapour, with the resulting phenomena of rain, hail, electric discharge, &c., and this column of air to take on a rapid spiral movement, which it naturally does, then the downward extension of this spiral movement to the ground provides the tornado. It is in respect to this thunderstorm origin that it differs from a whirlwind, the latter usually originating at ground level and not rising high enough to cause condensation in the very dry, hot air in which it occurs. The radius of action of the tornado may not be much greater than the few yards covered by an ordinary whirlwind, and rarely exceeds one-fourth of a mile, but what it lacks in area it more than makes up in intensity. From the cyclone it differs in the area affected, but, nevertheless, it is not to be regarded as a miniature cyclone. The tornado is a part of one thunderstorm; the cyclone is a vastly wider circulation of the air set in motion, at all events when of tropical origin, by the prevalence over a considerable area of the earth's surface of conditions which may be incidentally indicated by the occurrence of thunderstorms and even tornadoes in isolated parts of it. Some of the primary essentials to these conditions would be heat and atmospheric humidity above normal. It may be suggested, too, that the vertical temperature gradient would provide a means of definitely separating the two. In the tornado or thunderstorm the rising air must at any level, except possibly near the top, be warmer than the surrounding air at the same level, while above the cyclone the air soon becomes actually colder than at the same levels in the surrounding anticyclones. This is, of course, only another way of saying that a steep vertical temperature gradient is favorable for the occurrence, first of thunderstorms, and ultimately of cyclones.

Typical tornadoes are commonly supposed to be confined to North America. This is only true to the extent that they are undoubtedly more frequent and probably more violent there than elsewhere. Australian experience provides many genuine examples, but owing to the sparse population and the character of the storms themselves they have not yet been the subjects of very accurate scientific observation. Judging by the records available, New South Wales and Victoria appear to be the States most liable to these visitations. In New South Wales they are most frequent in the summer, occurring only in connexion with monsoonal depressions; in Victoria they seem quite as liable to occur in connexion with strong Antarctic low-pressure systems, and the numbers do not therefore show the same marked preferences for the summer season.