IT ALWAYS RAINS
All are familiar with the answer given by the native of Skye to the irate
tourist on that island, who, for the sixth day drenched, asked the
question: "Does it always rain here?" "Na!" answered the workman, without
at all understanding the joke; "feiles it snaas" (sometimes it snows).
Yet, strange to say, the tourist's question has been answered in the
affirmative in every place where a cloud is overhead, visible or
invisible.
Whenever a cloud is formed, it begins to rain; and the drops shower down
in immense numbers, though most minute in size--"the playful fancies of
the mighty sky."
No doubt it is only in certain circumstances that these drops are
attracted together so as to form large drops, which fall to the earth in
genial showers to refresh the thirsty soil, or in a terrible deluge to
cause great destruction. But when the temperature and pressure are not
suitable for the formation of what we commonly know as the rain, the fine
drops fall into the air under the cloud, where they immediately evaporate
from their dust free-surfaces, if the air is dry and warm. This is, in
other words, the decay of clouds.
It is a curious fact that objects in a fog may not be wetted, when the
number of water-particles is great. It seems that these water-particles
all evaporate so quickly that even one's hand or face is not sensible of
being wetted. The particles are minutely small; and they may evaporate
even before reaching the warm skin, by reason of the heated air over the
skin.
There is a peculiarly warm sensation in the centre of a cumulus cloud,
especially when it is not dense. A glow of heat seems to radiate from all
points. Yet the face and hands are quite dry, and exposed objects are not
wetted; but it is really _always raining_. That is a curious discovery.
It is radiant heat that is the cause of the remarkable result. The rays of
the sun, which strike the upper part of the cloud, not only heat that
surface but also penetrate the cloud and fall on the surface of bodies
within, generating heat there. These heated surfaces again radiate heat
into the air attached to them. This warm air receives the fine raindrops
in the cloud, and dissolves the moisture from the dust-particles before
the moisture can reach the surfaces exposed. That a vast amount of radiant
heat rushes through a cloud is clearly shown by exposing a thermometer
with black bulb _in vacuo_. On some occasions, a thermometer would
indicate from 40 deg. to 50 deg. above the temperature of the air, thus proving
the surface to be quite dry.
These observations have been corroborated on Mount Pilatus, near
Lucerne--1000 feet higher and more isolated than the Rigi. The summit was
quite enveloped in cloud, and, though one might naturally conclude that
the air was dense with moisture, yet the wooden seats, walls, and all
exposed surfaces were quite dry. Strange to say, however, the thermometers
hung up got wet rapidly, and the pins driven into the wooden post to
support them rapidly became moist. A thermometer lying on a wooden seat
stood at 60 deg., while one hung up read only 48 deg. This difference was caused
by radiant heat.
It is well known that, when bodies are exposed to radiant heat, they are
heated in proportion to their size; the smaller, then, may be moist, when
the larger are dry by radiation. The effect of the sun's penetrating heat
through the cloud is to heat exposed objects above the temperature of the
air; and if the objects are of any size they are considerably heated, and
retain their heat more, while at the same time around them is a layer of
warm air which is quite sufficient to force the water-vapour to leave the
dust-particles in the fine rain.
Hence seats, walls, posts, &c., are quite dry, though they are in the
middle of a cloud. They are large enough to throw off the moisture by the
retained heat, or by the large amount of surrounding heat; whereas, small
bodies, which are not heated to the same degree and cannot therefore
retain their heat so easily, have not heat-power sufficient to withstand
the moisture, and they become wetted. Hence, by the radiant heat, the
large exposed objects are dry in the cloud; whereas small objects are
damp, and, in some cases, dripping with wet.
The fact is, then, that whenever a cloud overhangs, _rain is falling_,
though it may not reach the earth on account of the dryness of the stratum
of air below the cloud, and the heat of the air over the earth. So that on
a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is
really raining; but the drops, being very small, evaporate long before
reaching the earth. As Ariel sings at the end of "The Tempest" of
Shakespeare, "The rain, it raineth every day." It rains, but much of the
melting of the clouds is reproduced by a wonderful circularity--the
moisture evaporating, seizing other dust-particles, forming
cloud-particles, falling again, and so on _ad infinitum_, during the
existing circumstances.