To understand the philosophy of this beautiful and often sublime
phenomenon, a few facts derived from observation and a long train of
experiments must be remembered.
1. Were the atmosphere every where at all times at a uniform
temperature, we should never have rain, or hail, or snow. The water
absorbed by it in evaporation from the sea and the earth’s surface
would descend in an imperceptible vapour, or cease to be absorbed
by the air when it was once fully saturated.
2. The absorbing power of the atmosphere, and consequently its
capability to retain humidity, is proportionally greater in warm
than in cold air.
3. The air near the surface of the earth is warmer than it is in
the region of the clouds. The higher we ascend from the earth, the
colder do we find the atmosphere. Hence the perpetual snow on very
high mountains in the hottest climate.
Now when, from continued evaporation, the air is highly saturated
with vapour, though it be invisible and the sky cloudless, if its
temperature is suddenly reduced by cold currents descending from
above or rushing from a higher to a lower latitude, its capacity to
retain moisture is diminished, clouds are formed, and the result is
rain. Air condenses as it cools, and, like a sponge filled with water
and compressed, pours out the water which its diminished capacity
cannot hold. What but Omniscience could have devised such an admirable
arrangement for watering the earth?