The irregular distribution of land and water, and the peculiar action of each in imparting the heat of the sun to the incumbent air,— the irregular distribution of plains and mountains, and their various effects in different positions and at different altitudes,—the distribution of heat effected by ocean currents,—all these tend to produce permanent derangements of climate and great irregularities in the weather. To these we must add what the astronomer calls disturbing actions of the second order,—effects of the disturbances themselves upon the action of the disturbing agencies,—effects of the irregular winds upon the distribution of heat and rain, and upon the action of lands and seas, mountains and plains. Though such disturbances are comparatively insignificant in the motions of the planets, yet in the weather they are often more important than the primary causes.
The aggregate and permanent effect of all these disturbing causes, primary and secondary, is seen in that irregular distribution of climates, which the tortuous isothermal lines and the mottled raincharts illustrate. The isothermal lines may be regarded as the topographical delineations of that bed of temperatures down which the upper atmosphere flows from the equator toward the poles, till its downward tendency is balanced by the centrifugal force of its eastward motion. This irregular bed shifts from month to month, from day to day, and even from hour to hour; and the lines that are drawn on the maps are only averages for the year or the season.
In the midst of these irregular, but continuous agencies, the rain introduces a peculiar discontinuity, and turns irregularity into discord. We have shown that the rain is an immediate cause of wind; but how is the rain itself produced? For so marked an effect we naturally seek a special cause; but no adequate single cause has ever been discovered. The combination of many conditions, probably, is necessary, such as a peculiar distribution of heat and moisture and atmospheric movements; though the immediate cause of the fall of rain is doubtless the rising, and consequent expansion and cooling, of the saturated air.
The winds that blow hither and thither, vainly striving to restore equilibrium to the atmosphere, burden themselves with the moisture they absorb from the seas; and this moisture absorbs their heat, retards their motion, and slowly modifies the forces which impel them. Now when the saturated air, extending far above the surface of the earth, and carried in its movements still higher, is relieved of an incumbent weight of air, it becomes rarefied, and its temperature and capacity for moisture are simultaneously diminished; its moisture, suddenly precipitated, appears as a cloud, the particles of which collect into rain-drops and fall to the earth. Thus the air suddenly loses much of its weight, and instead of restoring equilibrium to the troubled atmosphere, it introduces a new source of disturbance. Though the weight of the air is diminished by the fall of rain, yet the bulk is increased by the expansive force of the latent heat which the condensed vapors set free. Thus the rainy air expands upwards and flows outwards, and no longer able to balance the pressure of the surrounding air, it is carried still higher by inblowing winds, which rise in turn and continue the process, often extending the storm over vast areas. The force of these movements is measured partly by the force of latent heat set free, and partly by the mechanical power of the rain-fall, a very small fraction of which constitutes the water-power of all our rivers. Such a fruitful source of disturbance, generated by so slight an accident as the upward movement of the saturated air, expanded by its own agency to so great an extent, so sudden and discontinuous in its action, so obscure in its origin, and so distinct in its effects,—such a phenomenon defies the powers of mathematical prediction, and rouses all the winds to sedition.
A storm not only disturbs the lower winds, but its influences reach even to the upper movements. The sudden expansion and rising of the rainy air delay these movements, which afterwards react as violent winds.
The forces stored away by the gradual rise of vapor and its absorption of heat, and then suddenly exhibited in a mechanical form by the effects of rain, afford an illustration of that principle of conservation and economy of power, of which there are so many examples in modern science. No power is ever destroyed. Whether exhibited as heat or mechanical force, in the products and forces of chemical or of vital action, in movement or in altered conditions of motion,—whether changed by the growth of plants into fuel or into food, and converted again to heat by combustion or by vital processes, and brought out as mechanical power in the steam-engine or in the horse,—it is still the same power, and is measured in each of its forms by an invariable standard. It first appears as the heat of the sun, and a portion escapes at once back into space, while the rest passes first through a series of transformations. A part is changed into moving winds or into suspended vapor, and a part into fuel or food. From conditions of motion it is changed into motion; from motion it is changed by friction or resistance into heat, electric force, molecular vibrations, or into new conditions of motion, and passing through its course of changes, it remains embroiled in its permanent effects or escapes into space as heat.
Though mechanical science will probably never be able to predict the beginning or duration of storms, it will yet, doubtless, be able to account for all their general features, and for such distinct local peculiarities as observation may determine. Great advancement has already been made in the determination of prevailing winds and in the study of storms. Two theories have been brought forward upon the general movements of storms; both have been proved, to the entire satisfaction of their advocates, by the storms themselves; and probably both are, with some limitations, true. The first of these theories we have already described. According to it, the winds move inward toward the centre of the storm; according to the other theory, they blow in a circumference around the centre.
Observations upon storms of small extent, such as thunder-storms or tornadoes, show very clearly that the winds blow toward the stormy district. But when observations are made upon the winds within the district of such extensive storms as sometimes visit the United States, the directions of the wind are found to be so various, that the advocates of either theory, making due allowance for local disturbances, can triumphantly refute their adversaries. In such storms there are doubtless many centres or maxima of rain, and whether the wind move around or toward these centres, it would inevitably get confused.
The opinion, that the winds move around the central point or line of the storm, was strenuously maintained by the late Mr. Redfield, whose activity in his favorite pursuit has connected his name inseparably with meteorology. Others have maintained the same opinion, and the rotatory motion of the tropical hurricanes is offered as a principal proof. It is obvious from the causes of motion already considered, that, if the air is carried far, by its tendency toward a rainy district, it will acquire a secondary relative motion from its change of latitude; and this, in our hemisphere, if the air move toward the south, will be westward,—if toward the north, eastward. Hence the motion of the air from both directions toward a stormy district is deflected to the right side of the storm; and this gives rise to that motion from right to left which is observed in the hurricanes of the northern hemisphere.
To suppose, as many do, that regular winds, arising from constant and extensive causes, can come into bodily conflict and preserve their identity and original impetus for days, without immediate and strongly impelling forces to sustain their motion, implies a profound ignorance of mechanical science, and is little better than those ancient superstitions which gave a personal identity to the winds. The momentum of ordinary winds is a feeble force in comparison with those forces of pressure and friction which continually modify it. Hence sudden changes in the direction and intensity of winds must primarily arise from similar changes in these forces. But there are no known forces which change so suddenly, except the pressure and latent heat of suspended vapor; and therefore the fall of rain is the only adequate known cause of those storm-winds which, interpolated among the gentler winds, keep the atmosphere in perpetual commotion.