CIRRUS
A CLOUD is sometimes defined as any visible mass composed of small
particles of ice or water suspended in the air, and formed by
condensation from the state of vapour. As a general rule this is exact
enough, but under certain circumstances it is possible to have the
particles so small, and so thinly scattered, that it is not fully
satisfied. The resulting mass may not be actually visible. The presence
of the condensed particles may be indicated by nothing more than a
slight whitening of the blue sky, or by the formation about the sun or
moon of bright circles of light known as halos. If such a halo appears,
it is generally a phenomenon of brief duration. Sometimes the circle
breaks and becomes incomplete by the passing away of the thin patch of
cloud, sometimes the cloud increases in density until the rings are
destroyed.
The thinnest variety of this halo-producing structure is quite
invisible to the eye. It is so thin as to have no distinctly noticeable
effect upon the colour of the sky, but the optical results of its
presence may be very remarkable. Highly complicated systems of rings
are sometimes produced, the rings, as a rule, falling into two groups.
The commonest form has the sun (or moon) in the centre, and a circle
of pale light at a distance of about 22 degrees. Larger rings are seen
less frequently, which have an angular radius of about 46 degrees, and
as a rule have the sun situated on the ring itself. In Plate 1 we have
a part of such a great halo. The camera was directed towards the east,
and tilted upwards at an angle of about 40 degrees. The sun was behind
the camera, in the south-west, and the ring could be traced right up to
it on either side.
At the same time the sun was surrounded partially by a halo of the
more ordinary type, which was brightly coloured, making an effective
contrast to the dull white of the greater ring. The phenomenon did
not last more than half an hour, and the changes in its appearance
coincided with a growing density of cloud. When first noticed the
great ring was alone, and the sky was of a full blue, but a silvery
film came gradually up from the south-west, and the smaller and
brighter halo flashed out as the delicate curtain came near the sun.
Slowly the cloud spread to the north-east, gathering density from the
opposite point of the compass; and by the time the ordinary halo was at
its best, the great white ring had completely vanished.
[Illustration: Plate 1.
PART OF A GREAT HALO.]
[Illustration: Plate 2.
PART OF A SOLAR HALO.]
These circles, and the bright spots called mock-suns or mock-moons
which often accompany them, can all be explained on the assumption
that their cause is the passage of light through a veil composed of
hexagonal crystals of ice. The simple halo of 22 degrees radius is
common in most parts of the world, being very generally formed by the
film of high cloud which marks the advancing edge of a cyclonic cloud
system. A portion of one is shown in Plate 2, in which the rudimentary
fibrous structure of the sheet of cloud is distinctly seen. Halos of
this sort are frequently coloured, often most brilliantly so; but the
tints are seldom noticed unless a black mirror is used. They are
sometimes quite as bright as those of an ordinary rainbow, but instead
of being projected upon a background of dark rain-clouds, they are
seen against a part of the sky which is near the sun, and therefore
exceptionally bright.
The red is always on the inside of the ring, the violet outside,
thereby distinguishing them at once from the so-called coronæ, which
are formed around the sun or moon when shining through a sheet of alto
or other lower cloud made up of liquid particles. In these the radius
of the rings is much less, and the red is on the outside, the violet
actually touching the central luminary.
The cloud which produces halos is called cirro-nebula. It is much
thinner, and on an average higher than cirro-stratus. Mr. Ley named
it cirro-velum (or cirro-veil), but cirro-nebula has now got to be
fairly well understood. It sometimes appears and disappears in a
curious manner, showing that it occurs in patches, which drift about or
which keep forming and melting away, only to repeat the process. If,
however, it forms part of an advancing cyclone fringe, then the sky
gets whiter and whiter, until it is covered with a sheet of undoubted
cirro-stratus. This process of growing density is shown in progress in
Plate 3.
[Illustration: Plate 3.
CIRRO-NEBULA CHANGING TO CIRRO-STRATUS.]
Cirro-nebula, as we shall call it, floats at very great altitudes in
temperate regions; but in polar latitudes, where the optical phenomena
peculiar to it are most brilliant and diversified, it seems probable
that the ice dust is much lower down, even in actual proximity to the
ground. In England its height varies greatly with the time of year, and
other circumstances, but mounts up in summer to such altitudes as nine
miles or more; the greatest height yet recorded being 9·6 miles, or
about 15,500 metres, at Exeter.
The change from cirro-nebula to cirro-stratus is generally accompanied
by the formation of a distinct fibrous structure, easily observable by
the black mirror. This is not really a new feature, but only a further
development of a structure already existing, but too minute to be
easily seen. True halo-producing cirro-nebula may usually be shown to
possess more or less of a fibrous texture in an indirect way, which is
worth a brief description.
In order to observe the spots on the sun and other features of the
solar surface, it is a common practice to hold a white screen, say,
about a foot from the eyepiece of a telescope, while the instrument
is pointed to the sun. An image, considerably magnified, is thus
projected on to the screen, and the solar details can be studied with
ease and safety. If thin clouds drift before the sun, their images are
similarly projected as they pass across its disc, and it is possible
thus to detect not only the fibrous texture but also the movement of
cirro-nebula.[2]
The change into cirro-stratus is also attended by a marked fall in
altitude, but whether this is due to an actual descent of the cloud
particles, or to a downward spread of the conditions which give rise
to them, cannot at present be definitely settled. The balance of
probability points very strongly towards the downward spread of the
conditions. It is likely that the clouds, particularly the cyclonic
specimens, are wedge-shaped, and that as they pass overhead we see
first the thin edge, and later on the thicker parts, which project
much lower down. This is just one of those many minor problems in
cloud mechanics which we are not able to solve from the scanty data on
record.
Occasionally cirro-nebula breaks up into little detached
semi-transparent cloudlets, all of them exceedingly thin, and showing
a complicated mottling, resembling, on a minute scale, the ripple
clouds of much lower altitudes. Such a sky is depicted in Plate 4, but
no reproduction can possibly do justice to the minute and delicate
features of the real thing. The arrangement of the faint markings was
in a state of continual flux, curiously similar to the ever-changing
aspect of the sun’s photosphere when seen under adequate power. Some
parts of the cloud stratum would at one moment break up into distinct
granules arranged in complicated patterns, other parts would assume a
fibrous texture, and yet other places would show a continuous smooth
sheet. In a minute or two all would be changed--the smooth part
granulated, the fibres vanished, and the granules fused together, and
so on, no two of a series of photographs representing the same details.
[Illustration: Plate 4.
CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.]
These changes of form continued until the whole was hidden from view
by a veil of much lower stratiform cloud, one advance portion of which
is shown. Plate 4 does not represent a type or a distinct variety of
cloud. It is an intermediate form, or a temporary condition, showing
cirro-nebula in the act of changing into cirro-cumulus, or possibly
cirro-stratus.
Cirro-nebula itself, in its simpler form, is, however, a distinct type.
It is true that it never persists over one locality for more than an
hour or two without passing into some denser form, but while it lasts
its features are so distinctive, and the optical phenomena to which it
gives rise are so striking and significant, that it is a matter for
surprise that it should in the International system have been relegated
to the position of a subordinate variety of cirrus. It is more nearly
related to cirro-stratus, but is sufficiently distinct from that to
deserve at least specific rank.
True typical cirrus must have a plainly shown fibrous structure. The
fibres may cross and interlace, they may radiate in fan-like manner,
or they may curl and twist like a well-trimmed ostrich feather. The
clouds so formed must not be arranged in a continuous level sheet, or
they at once become cirro-stratus, and it is impossible to invent a
definition which will mark the exact limits of either type. Typical
cirrus consists of detached clouds. They cast no shadows on the
landscape, for the simple reasons that they are semi-transparent and
their component parts too narrow. If the sun is shining down obliquely
through the naked boughs of a tall tree, it will be seen that the
lowest twigs cast fairly sharp shadows on the ground, but that even
these are bordered by a fading rim; the twigs further up cast no sharp
shadows, but broader faint bands of shade; while the topmost boughs
cast no shadows which can clearly be identified. In other words, the
more distant the narrow twig is from the ground the narrower the real
shadow or umbra, and the broader the penumbra becomes, until when the
distance is sufficient the shadow is all penumbra. Cirrus filaments
throw nothing but a faint penumbra. Indeed, it is only when they lie
in the earth’s shadow, and stand against the background of a faintly
lighted sky, that they show any sign of shadow even on themselves.
There is no doubt that they are composed of particles of ice. They are
formed at altitudes where the thermometer must be many degrees below
freezing-point, and not a few of the thinner examples show fragmentary
halos like those of cirro-nebula.
Their actual altitudes are very variable, being greater in summer than
in winter, and reaching a maximum for any given station after a long
spell of hot weather. Exact measurements have not yet been made in
tropical latitudes or in polar regions, but there is every reason to
expect that the upper limit of cirrus for equatorial districts will
be found to be much higher than in the temperate zones where actual
observations have been made. In places nearer to the Arctic Circle it
is also almost certain that the altitudes will be less.
In the New England states, as shown by the Blue Hill observations, the
maximum altitude for summer was found to be little under 15,000 metres.
At Upsala, in Sweden, it was 13,300 metres. The average altitudes at
the same observatories were, respectively, about 9900 and 8800 metres.
At Exeter the writer’s own measurements give an average for the summer
months of 10,200 metres, with a minimum rather lower than was the case
in America or Sweden, and with a maximum far above the foreign values.
In winter cirrus certainly comes much lower down, but the number of
observations is fewer.
[Illustration: Plate 5.
HIGH CIRRUS.
(_Cirrus Excelsus._)]
The loftiest variety of cirrus appears in the afternoon in very hot
weather, sometimes quite late in the evening; and in autumn it is by no
means a rare event for it to suddenly form just when the sunset colours
are fading, or even after they have paled into twilight. Under such
circumstances it stands out of a shining silvery grey colour against
the background of the darkening sky. A specimen of it is shown in Plate
5, which shows its extreme slightness of form and delicacy of texture.
Sometimes it remains visible so long after the stars have begun to
show as to give the idea that it is self luminous, and the illusion
is certainly very strong. The writer has noted several instances in
which it was plainly visible, like a silvery curtain, though the sky as
a whole was so dark that stars like the five brightest points of the
Great Bear could be seen through the cloud, and much smaller stars down
to the third and fourth magnitude were plainly visible in the clear
intervals. It has sometimes been called luminous cloud, and Mr. Ley
estimated its altitude at upwards of 90,000 metres; but if we think of
it as reflecting the light of the distant colourless twilight there is
no need to regard it as anything fundamentally different from other
clouds, or to assume a greater altitude than we know to have been the
case. The specimen figured occurred in the early afternoon on June
12, 1899, at Exeter, and careful measurements of its altitude were
made. This worked out as 17·02 miles, or more than 27,000 metres, a
value so much greater than all other measurements of the kind that it
was only after most careful verification and reference to duplicate
records that it could be accepted. It differs in several ways from the
lower varieties, being thinner, more glistening, and in every way more
delicate. A suitable distinctive name would be high cirrus, or cirrus
excelsus.
[Illustration: Plate 6.
WINDY CIRRUS.
(_Cirrus Ventosus._)]
Lower down by thousands of metres come the feathery masses of typical
windy cirrus, such as are shown in Plate 6. Indeed, in cold winter
weather they occur within three or four thousand metres of the ground.
In the instance figured the wind was blowing from left to right, and
the clouds were travelling swiftly. The upper filaments appeared to be
repeatedly torn away from the main masses, while the long faint streaks
which trail below and behind are evidently due to streams of fine
particles falling from the main centres of condensation into a less
rapidly moving stratum below. There is no room for doubt that these
clouds, like others of a similar order, are formed by a direct passage
from the vapour to the solid, or that the fibres are made of minute
snowflakes. The condensation is evidently attended by rapid movements,
which draw out the cloud, as fast as it is formed, into long curving
lines which mark lines of motion. The variety is always, therefore, an
indication of strong winds and rapid eddying movements in the region
in which it occurs. Such strong disturbances overhead almost always
accompany similar but less intense movements at the ground-level, and
when they do not accompany them they precede them. The cloud is well
named windy cirrus, which may be converted into a specific name, cirrus
ventosus.
The next variety we come to (Plate 7) is in some ways rather similar.
It is, however, thinner, more delicate, and is entirely composed
of fine threads, which are more systematically arranged. Generally
there is a bundle, or several bundles, of long parallel fibres, which
form, so to say, the quill of the feather, with numbers of shorter
threads branching out from them at various angles. Cirrus ventosus was
indicative of irregular movements in various directions; this variety
points also to complicated movements, but executed in accordance with
some sort of system, strangely complex and wonderfully ordered. The
specimen figured is the type of what Mr. Ley called cirro-filum, or
thread cirrus, and his name can hardly be bettered. It is a cloud of
summer, and occurs rather high up in the cirrus zone, but no actual
measurements can be quoted. It is fairly common, but not nearly so
frequent as the last.
[Illustration: Plate 7.
THREAD CIRRUS.
(_Cirro-filum._)]
A somewhat more familiar variety is shown in Plate 8. Little irregular
feathers of cirrus, from which long tapering streamers point downwards
in graceful curves, or else lag behind in the direction from which
the clouds have travelled. If clouds of this type are carefully
watched, it will soon be seen that each feathery head is a centre of
condensation, and that the tails or streamers are nothing else than
falling particles, which dwindle slowly away by evaporation, and which
gradually sink below the level of the heads. It is usual, in dealing
with cloud-forms like these, to speak of air-currents of different
velocities almost as if the winds at different levels were as clearly
separated as oil and water, or even air and water. This can hardly
be the case, for if such a thing should occur as an air-current of
one velocity flowing over another of less speed, or of a current in
one direction over another moving in a different course, the two must
inevitably mix at their junction, and in a very short time the passage
from the lower current to the upper one would be quite gradual. No
doubt we can often observe two, three, or more layers of cloud moving
in different directions; but if we were to send up a balloon, it would
be rare indeed to find its direction of horizontal movement changed in
a few metres of ascent. Different and distinct air-currents are often
invoked to explain cloud-forms quite unnecessarily. It is far more
likely that the differential movements involved in the explanation of
the features of these cirrus varieties are due to increased velocity
with greater altitude, to progressive change of direction, to irregular
eddies, or to the interaction of ascending and descending convection
currents. Indeed, it is probable that careful study of the growth and
decay of these clouds will, in time, lead to a clearer understanding
of atmospheric movements, and so enable us to say more precisely why
they are as we see them to be. The variety shown in Plate 8 is rare
except in combination with other forms. It might well be termed tailed
cirrus or cirrus caudatus.
[Illustration: Plate 8.
TAILED CIRRUS.
(_Cirrus Caudatus._)]
The form of cirrus shown in Plate 9 is far more frequently seen than
either of those which have been described. In this the fibrous texture
is very imperfect, and the cloudlets show a tendency to arrange
themselves in a kind of ribbed structure in two directions almost at
right angles to each other. But this last is an accidental feature
of the particular example, and not in any way a specific character
of the cloud. The reason for regarding it as a distinct variety is
the total absence of sharply defined lines, not only the heads of
condensation, but even the long streamers attached to them being
uniformly hazy and ill-defined. It is a form of cirrus which comes at
all seasons, but most frequently in summer; it moves always with great
slowness, indicating a quiet atmosphere free from disturbance of any
kind. The conditions necessary for its appearance are a nearly uniform
distribution of pressure over a considerable area, chequered by little
shallow depressions of some trifling fraction of an inch. In hot
weather these are the conditions under which thunder-storms develop,
and this hazy cirrus, or cirrus nebulosus, may be taken as a certain
sign of such an atmospheric state.
[Illustration: Plate 9.
HAZY CIRRUS.
(_Cirrus Nebulosus._)]
So far as permanency of form is considered, hazy cirrus is one of the
most persistent, and affords a marked contrast to the species shown in
Plate 10, which represents the most fugitive. Five minutes before the
photograph was taken the same part of the sky was a deep, clear blue,
without any trace of cloud. Suddenly a few short curling wisps made
their appearance. These rapidly increased in number, until a delicate
filmy network extended over the greater part of the field of view. But
while the camera was being adjusted for an exposure, part of the net
had broken up into the granular structure shown in the lower part of
the photograph. The granulation rapidly spread through the net, almost
as if the fibres had been curdled, and five minutes later the whole had
been converted into a patch of cirro-cumulus which soon fused into a
uniform sheet. Meanwhile the same series of phenomena were taking place
in other parts of the sky.
[Illustration: Plate 10.
CHANGE CIRRUS.
(_Cirrus Inconstans._)]
On other occasions exactly the same set of events have been seen to
follow each other in the inverse order. Beginning with a fairly even
sheet, this broke up into granules, and they in turn seemed to be
frayed out into short hazy and wavy fibres which slowly melted away.
Clearly we have here to do, not with a distinct type of cloud, but
rather with the first step towards the formation of one, or the last
stage in the life of one which is drying up. But sometimes the life of
the cloud is so short that it never passes beyond this first stage;
and it is by no means a universal rule for a growing sheet of cirrus
to pass through this stage at all. It therefore represents a peculiar
state of instability, and requires a name of its own. Sometimes patches
of it will come and go in an apparently capricious manner for an
hour or more before permanent condensation is effected or before the
sky finally clears. But this is a rare event, since the slow change
of conditions which has brought the stratum of air to the unstable
condition is generally progressive, and instead of stopping at the
critical point, goes beyond it, with the result that the condensation
grows or the cloud disappears entirely. Change cirrus, or cirrus
inconstans, would be an appropriate name for a kind of cloud which is
so plainly indicative of instability.
The critical condition referred to is, of course, that in which a
particular stratum of air is just saturated, or is just on the point
of forming visible cloud. If any cause is brought to bear on such a
stratum which brings about even slight cooling, cloud must be produced;
and, conversely, anything which results in the slightest heating must
cause it to disappear. The shortness and haziness of the fibres, and
the fact that they gather themselves into granules, shows that the
cloud is formed in a stratum of air which is either still, or is moving
as a whole, without any of those differential movements which seem to
be necessary for the longer fibrous details.
The causes which may bring about the local cooling and heating are easy
to understand when we remember how the air will be affected by the
uneven contours of the ground. As it passes over hill and valley the
up-and-down movements of the lower layers, or even the disturbances
caused by passing over a wood or clump of trees, all must be propagated
upwards. Each disturbance must slowly spread laterally and diminish
vertically, so that it will reach the cirrus zone as a broad and
gentle dome-like oscillation. Suppose now a series of such slight
upheavals to reach the critical level. The passage of the waves will
mean alternate expansion and compression. Expansion means cooling, and
therefore cloud-production; compression means heating, and therefore
the destruction of cloud.
From the most transient form of cirrus we pass, in Plate 11, to the
most persistent and probably the most frequent. It occurs in detached
masses which have very variable forms but are wholly fibrous, with the
details arranged in a very irregular manner. The example figured was
taken in the evening during a long spell of fine weather. If such a
cloud is watched, its permanence of detail is very striking, and must
be due to a persistence of slow eddying movements and to a continual
renewal and waste of the component particles of each wisp. This is
the kind of cirrus selected generally as the type of cirrus, and the
selection is a good one. Common cirrus, or cirrus communis, it should
be called. Settled conditions and fine weather are its usual attendants.
[Illustration: Plate 11.
COMMON CIRRUS.
(_Cirrus Communis._)]
We next come to a variety which is anything but a harbinger of good,
namely, the long stripes or bands of cirrus which stretch outwards
from the margin of the cloud canopy of a cyclonic storm. In some ways
these appendages to the great nimbus resemble the strips of cirriform
cloud which fringe the summit of a thunder-cloud. They look as if they
must have been formed by the blowing away, by a rapid wind, of the top
of an uprising column of vapour-charged air. Their main outline may
thus be easily accounted for, but we have only to study their detailed
structure for a few minutes to feel that they really present a problem
of a very high order. Plate 12 shows a fairly simple example, but Plate
13 represents a cloud of very great complexity. To take this last the
camera was tilted upwards at an angle of 45 degrees, so that the top of
the picture is not far from the zenith. The wonderful plume of cloud
rose from the southern horizon, and ended in a great sheaf of fibres
and films spread out like a partly opened fan whose edge was only about
50 degrees above the northern horizon. Its length as it passed overhead
lay between a point a little east of south to a little west of north;
and the broad band moved as a whole, without any marked internal
changes, from the south-west towards the north-east. The weather was
very unsettled. A long procession of cyclones had been sweeping along
our western shores, and the barometer was just beginning a fresh and
rapid fall. During the ensuing night a heavy gale burst over the south
of England.
[Illustration: Plate 12.
BAND CIRRUS.
(_Cirrus Vittatus._)]
[Illustration: Plate 13.
BAND CIRRUS.
(_Cirrus Vittatus._)]
The whole phenomenon was highly characteristic. These great bands with
the divergent striation might well be known as storm bands, from their
almost invariable connection with the violent atmospheric movements
to which they are most probably due. Plate 12 shows a much less
dangerous variety of the same species, which is distinguished from it
by the comparative absence of internal detail and by the curled ends.
Clouds of this character have sometimes been called cirro-filum, but
a comparison of the plates with the typical cirro-filum of Plate 7
will show that there is little resemblance; and the attendant weather
is also in marked contrast, both of which facts imply a fundamental
difference in the causes to which their features are due. Banded or
ribboned cirrus is the name which they immediately suggest, and this
may be rendered cirrus vittatus.
This ends our survey of cirrus clouds. Any one who compares the plates
so far given will see that they represent forms so diverse that it is
impossible to avoid the conclusion that the conditions under which they
are produced must differ not only in degree but also in kind. What
those conditions are we have attempted here and there to suggest, but
in no case can we feel that the explanation has been at all complete.
In some cases, notably the last, we are face to face with such
complicated details that it is hopeless to attempt to explain them in
the present state of our knowledge. Fact upon fact must be accumulated
until we can give their history from their earliest beginnings; and
far more accurate and detailed knowledge of the attendant atmospheric
conditions must be acquired before we can hope to rob such elaborate
structures of their present mystery.
This requires the co-operation of many eyes and many minds, and exact
specific names must be an essential preliminary. Those which have been
suggested in these pages are--
1. Cirro-nebula, or Cirrus haze.
2. Cirrus excelsus, or High cirrus.
3. Cirrus ventosus, or Windy cirrus.
4. Cirro-filum, or Thread cirrus.
5. Cirrus nebulosus, or Hazy cirrus.
6. Cirrus caudatus, or Tailed cirrus.
7. Cirrus vittatus, or Band cirrus.
8. Cirrus inconstans, or Change cirrus.
9. Cirrus communis, or Common cirrus.
FOOTNOTE:
[2] The telescope with which these observations have been made is a
6·8-inch refractor equatorially mounted.