DECAY OF CLOUDS
From the earliest ages clouds have attracted the attention of observers.
Varied are their forms and colours, yet in our atmosphere there is one law
in their formation. Cloud-particles are formed by the condensation of
water-vapour on the dust-particles invisibly floating in the atmosphere,
up to thousands--and even millions--in the cubic inch of air.
But observers have not directed their attention so much to the decay of
clouds--in fact, the subject is quite new. And yet how suggestive is the
subject!
The process of decay in clouds takes place in various ways. A careful
observer may witness the gradual wasting away and dilution into thin air
of even great stretches of cloud, when circumstances are favourable. In
May 1896 my attention was particularly drawn to this at my manse in
Strathmore. In the middle of that exceptionally sultry month, I was
arrested by a remarkable transformation scene. It was the hottest May for
seventy-two years, and the driest for twenty-five years. The whole parched
earth was thirsting for rain. All the morning my eyes were turned to the
clouds in the hope that the much-desired shower should fall. Till ten
o'clock the sun was not seen, and there was no blue in the sky. Nor was
there any haze or fog.
But suddenly the sun shone through a thinner portion of the enveloping
clouds, and, to the north, the sky began to open. As if by some magic
spell there was, in a quarter of an hour, more blue to be seen than
clouds. At the same time, near the horizon, a haze was forming, gradually
becoming denser as time wore on. In an hour the whole clouds were gone,
and the glorious orb of day dispelled the moisture to its thin-air form.
This was a pointed and rapid illustration of the decay from cloud-form to
haze, and then to the pure vapoury sky. It was an instance of the
_reverse_ process. As the sun cleared through, the temperature in the
cloud-land rose and evaporation took place on the surface of the
cloud-particles, until by an untraceable, but still a gradual process
through fog, the haze was formed. Even then the heat was too great for a
definite haze, and the water-vapour returned to the air, leaving the
dust-particles in invisible suspension.
But clouds decay in another way. This I will illustrate in the next
chapter on "It always rains."
What strikes a close observer is the difference of structure in clouds
which are in the process of formation and those which are in the process
of decay. In the former the water-particles are much smaller and far more
numerous than in the latter. While the particles in clouds in decay are
large enough to be seen with the unaided eye, when they fall on a properly
lighted measuring table, they are so small in clouds in rapid formation
that the particles cannot be seen without the aid of a strong magnifying
glass.
Observers have assumed that the whole explanation of the fantastic shapes
taken by clouds is founded on the process of formation; but Dr. Aitken has
pointed out that ripple-marked clouds, for instance, have been clouds of
decay. When what is called a cirro-stratus cloud--mackerel-like against
the blue sky--is carefully observed in fine weather, it will be found that
it frequently changes the ripple-marked cirrus in the process of decay to
vanishing. Where the cloud is thin enough to be broken through by the
clear air that is drawn in between the eddies, the ripple markings get
nearer and nearer the centre, as the cloud decays. And, at last, when
nearly dissolved, these markings are extended quite across the cloud.
Whether, then, we consider the cases of clouds gradually melting away back
into their original state of blue water-vapour, or the constant fine
raining from clouds and re-formation by evaporation, or the transformation
of such clouds as the cirro-stratus into the ripple-marked cirrus, we are
forced to the conclusion that in clouds there is not always development,
but sometimes degeneration; not always formation, but sometimes decay.