Clouds are formed by the condensation of vapor at considerable but
various elevations in the atmosphere. Vapor is always invisible, clouds,
therefore, are not vapor but water, and consist of a fine watery powder,
the size of each particle being exceedingly minute; and consequently
they are so light that clouds formed of an accumulation of such
particles are readily borne forward by the winds. Clouds are sometimes
suddenly formed and as suddenly disappear, probably owing to sudden and
partial changes of temperature. When a considerable difference of
temperature prevails in the aerial currents which may come in contact
with the local atmosphere, a further condensation takes place, and the
particles of this fine watery powder unite into drops, and, becoming
heavier, fall to the earth in the form of _rain_, _hail_ or _snow_.