The cumulo-stratus cloud, which is the precursor of this kind of storm, can usually be observed only from one to eight hours, and, in some cases of the most violent kind, only about twelve hours before it will burst upon a place. Although these storms are the most dangerous and destructive—not unfrequently ending in tornadoes and hurricanes—the barometer is of no practical service in predicting it. This is explained by the fact that in such storms the plane of meeting of the two currents moves southward with its lower extremity, or region of lowest barometer, in front, while the plane itself is more or less inclined northward. Hence the barometer shows no change until this region of lowest barometer moves over it, when it suddenly falls g but it is then already in the most dangerous part of the storm, and its warning, therefore, comes too late; while the clouds, if properly observed, always give warning in time to provide against the dangers of such a storm.
Tornadoes.—This class of storms includes hailstorms, waterspouts, hurricanes, and all storms in which rotary and lateral motions are more or less combined. They are the most violent and destructive of all storms, as well as the most complicated and difficult to understand and explain. They are the offspring of progressive polar or summer storms, and in the temperate zone occur only during summer.
When in the development of a summer storm, as above described, the two conflicting currents attain a state of equal power or resistance, and thus balance each other, which is indicated when the dense cumulus clouds over the plane of conflict become stationary, then the storm is at its crisis. The air within the region of conflict is compressed and very sultry, and this condition is always felt before a tornado by persons within its area. If, now, during this critical stage of the storm, no topographic or other disturbance of its tension take place in its plane of meeting, a return oscillation of the polar current northward will set in, and the storm will gradually clear away. But if, in this crisis of the storm and during this high state of compression and resistance, either current becomes stronger, and forces back the other over some hill or valley, or if some other obstruction or configuration of the surface of the earth breaks the tension or disturbs the resistance between the two currents at any point, so that the polar current will sink as in a valley, then the tropical current will suddenly rush into this depression and generate a succession of violent whirling and zigzag motions along the diagonal of the two currents within the plane of conflict, as the waters of a dam would rush through a sudden break or depression in an embankment. This conclusion respecting the origin of tornadoes Prof. Blasius reached after his careful study of the West Cambridge tornado of 1851, and it was subsequently confirmed by the facts and phenomena connected with the tornado of Iowa and Illinois, in May, 1873, as obtained from the report of the
