By G. JOHNSTONE STONEY.
When the sky is occupied by light cirro-cumulus cloud, an optical
phenomenon of the most delicate beauty sometimes presents itself, in
which the borders of the clouds and their lighter portions are
suffused with soft shades of color like those of mother-of-pearl,
among which lovely pinks and greens are the most conspicuous. Usually
these colors are distributed in irregular patches, just as in
mother-of-pearl; but occasionally they are seen to form round the
denser patches of cloud a regular colored fringe, in which the several
tints are arranged in stripes following the sinuosities of the outline
of the cloud.
I cannot find in any of the books an explanation of this beautiful
spectacle, all the more pleasing because it generally presents itself
in delightful summer weather. It is not mentioned in the part of
Moigno's great _Repertoire d'Optique_ which treats of meteorological
optics, nor in any other work which I have consulted. It seems
desirable, therefore, to make an attempt to search out what appears to
be its explanation.
At the elevation in our atmosphere at which these delicate clouds are
formed the temperature is too low, even in midsummer, for water to
exist in the liquid state; and accordingly, the attenuated vapor from
which they were condensed passed at once into a solid form. They
consist, in fact, of tiny crystals of ice, not of little drops of
water. If the precipitation has been hasty, the crystals will, though
all small, be of many sizes jumbled together, and in that case the
beautiful optical phenomenon with which we are now dealing will not
occur. But if the opposite conditions prevail (which they do on rare
occasions), if the vapor had been evenly distributed, and if the
precipitation took place slowly, then will the crystals in any one
neighborhood be little ice crystals of nearly the same form and size,
and from one neighborhood to another they will differ chiefly in
number and size, owing to the process having gone on longer or taken
place somewhat faster, or through a greater depth, in some
neighborhoods than others. This will give rise to the patched
appearance of the clouds which prevails when this phenomenon presents
itself. It also causes the tiny crystals, of which the cloud consists,
to grow larger in some places than others.
Captain Scoresby, in his "Account of the Arctic Regions," gives the
best description of snow crystals formed at low temperatures with
which I am acquainted. From his observations it appears--(a) that
when formed at temperatures several degrees below the freezing point,
the crystals, whether simple or compound, are nearly all of
symmetrical forms; (b) that thin tabular crystals are extremely
numerous, consisting either of simple transverse slices of the
fundamental hexagon or, more frequently, of aggregations of these
attached edgewise and lying in one plane; and (c) that, according as
atmospheric conditions vary, one form of crystal or another largely
preponderates. A fuller account of these most significant observations
is given in the appendix to this paper.
Let us then consider the crystals in any one neighborhood in the sky,
where the conditions that prevail are such as to produce lamellar
crystals of nearly the same thickness. The tabular plates are
subsiding through the atmosphere--in fact, falling toward the earth.
And although their descent is very slow, owing to their minute size,
the resistance of the air will act upon them as it does upon a falling
feather. It will cause them, if disturbed, to oscillate before they
settle into that horizontal position which flat plates finally assume
when falling through quiescent air. We shall presently consider what
the conditions must be, in order that the crystals may be liable to be
now and then disturbed from the horizontal position. If this
occasionally happens, the crystals will keep fluttering, and at any
one moment some of them will be turned so as to reflect a ray from the
sun to the eye of the observer from the flat surface of the crystal
which is next him. Now, if the conditions are such as to produce
crystals which are plates with parallel faces, and as they are also
transparent, part only of the sun's ray that reaches the front face of
the crystal will be reflected from it; the rest will enter the
crystal, and, falling on the parallel surface behind, a portion will
be there reflected, and passing out through the front face, will also
reach the eye of the observer.
These two portions of the ray--that reflected from the front face and
that reflected from the back--are precisely in the condition in which
they can interfere with one another, so as to produce the splendid
colors with which we are familiar in soap bubbles. If the crystals are
of diverse thicknesses, the colors from the individual crystals will
be different, and the mixture of them all will produce merely white
light; but if all are nearly of the same thickness, they will transmit
the same color toward the observer, who will accordingly see this
color in the part of the cloud occupied by these crystals. The color
will, of course, not be undiluted; for other crystals will send
forward white light, and this, blended with the colored light, will
produce delicate shades in cases where the corresponding colors of a
soap bubble would be vivid.
We have now only to explain how it happens that on very rare occasions
the colors, instead of lying in irregular patches, form definite
fringes round the borders of the cloudlets. The circumstances that
give rise to this special form of the phenomenon appear to be the
following: While the cloud is in the process of growth (that is, so
long as the precipitation of vapor into the crystalline state
continues to take place), so long will the crystals keep augmenting.
If, then, a cloudlet is in the process of formation, not only by the
springing up of fresh crystals around, but also by the continued
growth of the crystals within it, then will that patch of cloud
consist of crystals which are largest in its central part, and
gradually smaller as their situation approaches the outside. Here,
then, are conditions which will produce one color round the margin of
the cloud, and that color mixed with others, and so giving rise to
other tints, farther in. In this way there comes into existence that
iris-like border which is now and then seen.
The occasional upsetting of the crystals, which is required to keep
them fluttering, may be produced in any of three ways. The cloudlets
may have been formed from the blending together of two layers of air
saturated at different temperatures, and moving with different
velocities or in different directions. Where these currents intermix,
a certain amount of disturbance will prevail, which, if sufficiently
slight, would not much interfere with the regularity of the crystals,
and might yet be sufficient to occasion little draughts, which would
blow them about when formed. Or, if the cold layer is above, and if it
is in a sufficient degree colder, there need not be any previous
relative motion of the two layers; the inevitable convection currents
will suffice. Another, and probably the most frequent, cause for
little breezes in the neighborhood of the cloudlets is that when the
cloudlets are formed they immediately absorb the heat of the sun in a
way that the previously clear air had not done. If they absorb enough,
they will rise like feeble balloons, and slight return currents will
travel downward round their margins, throwing all crystals in that
situation into disorder.
I do not include among the causes which may agitate the crystals
another cause which must produce excessively slight currents of air,
namely, that arising from the subsidence of the cloudlets owing to
their weight. The crystals will fall faster wherein cloud masses than
in the intervening portions where the cloud is thinner. But the
subsidence itself is so slow that any relative motions to which
differences in the rate of subsidence can give rise are probably too
feeble to produce an appreciable effect. Of course, in general, more
than one of the above causes will concur; and it is the resultant of
the effects which they would have separately produced that will be
felt by the crystals.
If the precipitation had taken place so very evenly over the sky that
there were no cloudlets formed, but only one uniform veil of haze,
then the currents which would flutter the crystals may be so entirely
absent that the little plates of crystals can fixedly assume the
horizontal position which is natural to them. In this event the cloud
will exhibit no iridescence, but, instead of it, a vertical circle
through the sun will present itself. This, on some rare occasions, is
a feature of the phenomenon of parhelia.
It thus appears that the occasional iridescence of cirrus clouds is
satisfactorily accounted for by the concurrence of conditions, each of
which is known to have a real existence in nature....--_Phil. Mag.,
July 1887._
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