Origin of Tornadoes

Dr. B. F. Duke, of Pascagoula, Miss., sends an account of a tornado observed by him in April, 1894, possibly at or near that place.

I was located on the edge of a track about a mile and a half wide, within which nearly everything was swept before the wind. It was a cloudy day, and thunder and rain had been observed all the afternoon in the west under very dark clouds. About 6 p. m. these clouds suddenly became very black in one place while everything around the observer was very calm and still. Soon a terrific roaring could be heard in the distance. As it approached, a low stratum of muddy cloud could be seen in the west, flying from northwest to southeast, while another stratum was coming up equally fast from the south, and puffs of wind from these two directions were alternately felt by the observer. All this occurred a little in advance of the dense black cloud, which was streaked with lightning, though not funnel-shaped so far as we could discern. When it (the tornado?) had passed by us, it was seen that the timber on the north side of the track was blown to the southeast while that on the south side fell toward the north, but in the center, or nearly so, it was piled in every direction and in the greatest possible confusion. In some places the wind seemed to have made all sorts of breaks and deflections, blowing in strips of a quarter of a mile or more, directly opposite to the general course which was nearly northeast. In some of these dashes, if we may so speak of them, it (the wind?) would appear to have been heavier than in the main body of the storm.

What conditions of the earth and air give rise to the south and the north west winds and the clouds that preceded the hurricane?

Is there not a strong attraction between them? When they meet, is not this affinity neutralized? Had these winds been coming from exactly opposite directions, would not the cyclone (tornado) have occurred throughout the whole length at the same moment? Does a tornado actually travel, or is its velocity to be reckoned by the acuteness, or obtuseness of the angle of these two approaching currents, which might be illustrated by two lines of battle advancing toward each other at the angle indicated, namely, one moving from south to north, the other from northwest to southeast; the time required for the two entire lines to meet depending upon the speed maintained?

In the United States when the weather map shows a center of low pressure, there is generally an extensive area of cold northerly winds and high pressure west of the center; but a region of warm southerly winds south and east of it. What conditions of the earth and air give rise to these winds? The only answer must be that the differences in density of different portions of the atmosphere cause these portions to be acted upon differently by the attraction of gravity and by the centrifugal force of the revolving atmosphere. Gravity pulls the denser air down, so that the cold northwest wind ​underruns and lifts up the warm, moist, southerly wind. Centrifugal force drives the denser cold air toward the equator, pushing the lighter, warm air out of its way, and forcing the latter upward and backward toward the polar regions. These are the principal mechanical conditions that give rise to the winds and clouds that precede such tornadoes as those in northern Mississippi on April 8-9 and 18-19, 1894.

On both these dates a cold, dry, northwest wind was advancing southward over the State as the front edge of an area of high pressure, while warm southerly winds were prevailing everywhere to the southward and eastward. The northwest winds were much stronger than the southerly winds, but they, themselves, did not constitute a tornado, nor could they have done the damage described without another auxiliary process. At the front of the area of northwest wind, where it ran under the south wind and lifted it up, as the nose of a plow lifts and turns the sod, there was formed a cloudy mass due to the rapidly uprising air. The buoyancy within such a cloud is very great. When once well formed, it may suck up the air beneath it with such violence as to form a waterspout over the ocean or a tornado over the land and the winds immediately below it are suddenly and greatly increased. It is these winds under the tornado cloud that do most of the destruction; they start toward the cloud as increasing northwest and south winds on the two sides of the track, but rapidly become deflected into circulating winds, under the cloud, extending sometimes even down as low as the ground itself. The individual clouds and whirls along the front of the northwest wind depend very much upon local irregularities, hills and valleys, rivers and ponds; in some cases there may be a long series of whirls simultaneously existing; at other times only one or two acquire any prominence; again, it may be as suggested by Dr. Duke, that there is an advancing front for the southerly winds as well as for the northwesterly, and that the whirl exists only at the one vertex where these two fronts intersect. All these and other cases may occur; but the last is certainly the least common because there is almost always a steady flow of southerly winds over a very large area of country and the front of the northwest wind is everywhere penetrating this and pushing under it simultaneously so that the southerly front has no independent existence.

We can not agree with the suggestion that there is a strong attraction between the northwest and the south winds, or that there is any neutralization of affinity; the winds represent simply two masses of air driven along the earth's surface by the pushing forces that are at work everywhere in the atmosphere and which are ultimately resolvable into two elementary forces, the attraction of gravitation and the centrifugal force of bodies that revolve with the diurnal rotation of the earth. These two forces will cause warm, moist air to push northward while cold, dry air is pushed southward and the tornadoes start in the narrow belt where the northerly winds push against the southerly.

In many cases a tornado involves a large mass of cloud and may be properly said to move bodily for quite a long distance along the earth's surface, as shown by its path of destruction. At other times a tornado rapidly dies out, but only to be quickly succeeded by another, so that the path of destruction is due to a series of newly-formed successive whirls. The axis of the whirl is oftentimes very much inclined to the earth's surface and it is possible that we may have violent whirls with horizontal axes; but they could not last very long.