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.
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