New volumes of the Encyclopædia Britannica > Volume 30, K-MOR

physics] meteorology
697
lot
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Clouds.
above 20 or 30 miles, where the temperature would become
absolute zero, according to the adopted law of diminution.
But the uncertainty of the various hypotheses as to the
physical properties of the upper atmosphere forbids us to
entertain any positive ideas on this subject at the present
time. If we define the outer limit as that point at which
the diffusion of gases inwards just balances the diffusion
outwards, then this limit must be determined not by the
hypsometric formula, but by the properties of gases at low
temperatures and pressures under conditions as
yet uninvestigated by physicists.
It is evident that the clouds are formed
from clear transparent air by the
condensation of the invisible moisture
therein into numerous minute globules of
water or crystals of ice and snow. Notwith-
— standing their transparency, these indivi¬
dual globules and crystals, when collected
— in large masses, disperse the solar rays by
reflexion to such an extent that direct light
— from the sun is unable to penetrate them,
and partial darkness results. In a general
survey of the atmosphere the geographical
distribution of the amount of cloudy sky
is important. When the solar heat falls
upon the surface of the cloud it is so
absorbed and reflected that, on the one
hand, scarcely any penetrates to the
ground beneath, while on the other
hand the upper surface of the cloud
becomes unduly heated. Even if
this upper surface is completely
evaporated, it may continually
be renewed from below, and,
moreover, the evaporated
moisture mixing with the
air renders it very much
lighter specifically than it
would otherwise be.
Hence the upper sur¬
face of the cloud
replaces the sur¬
face of the
ground and
•i- -5 -fc .7 .% .c\
t **Po^. *Pa.
S a. &. eJ¥.
Fig. 1.
of the ocean; the air in contact with it acquires a
higher temperature and greater buoyancy, while the
ground and air beneath it remain colder than they would
be in sunshine. The average annual cloudiness over
the globe is therefore intimately related to the density
and circulation of the atmosphere; it was first charted
in general terms by L. Teisserenc de Bort of Paris
about 1886. The manifold modifications of the clouds
impress one with the conviction that, when properly
understood and interpreted, they will reveal to us the
most important features of the processes going on in the
atmosphere. If the farmer and sailor can correctly judge
of the weather several hours in advance by a casual glance
at the clouds, what may not the professional meteorologist
hope to do by a more careful study ? Acting on this idea,
the author in 1868 asked from all of his correspondent
observers full details as to the quantity, kind, and direction
of motion of each layer of clouds; these were telegraphed
daily for publication in the bulletin of the Cincinnati
Observatory, and for use in the weather predictions made
at that time. Since January 1872 similar data have been
regularly telegraphed for the use of the U.S. Weather
Bureau in preparing forecasts, although the special cloud
maps that are compiled have not been published, owing to
the expense. These data were also published in full in the
Bulletin of the International Simultaneous Meteorological
Observations for the whole northern hemisphere during
the years 1875-84. The writer’s work on the U.S.
Eclipse Expedition to the West Coast of Africa in 1889-90
was wholly devoted to the determination of the height and
motions of the clouds by the use of a special form of the
marine nephoscope, whose use can easily be understood
from an examination of Figs. 2, 3, and 4. It consists
simply of a framework and mirror fitting upon the standard
compass of the navigator, and removable at a moment’s
notice if necessary. The use of this instrument is to be
strongly recommended, as it gives the navigator a means of
determining exactly the bearing of a storm centre at sea
by studying the lower clouds better than he can possibly
do by the observations of the winds alone. The im¬
portance of cloud study has been especially emphasized
by the International Meteorological Committee, which
arranged for a complete year of systematic cloud-work by
national weather bureaus and individual observatories
throughout the world from May 1896 to June 1897. In
this connexion Mr H. C. Clayton of Blue Hill Observa¬
tory has published a very comprehensive report on cloud
forms. The complete report by Professor F. H. Bigelow
on the work done by the U.S. Weather Bureau forms a
part of the annual report for 1899, and constitutes a
remarkable addition to our knowledge of the subject.
Some preliminary account of this work was published
in the American Journal of Science for December
1899.
In order to obtain the greatest benefit from the study
of the clouds, we must understand the laws and processes
involved in their formation; but as these constitute a
study in mechanics and physics of great complexity,
the results already obtained will be stated in
a subsequent section of this article as a part of
theoretical physical meteorology. Meanwhile the
following brief remarks relative to the kinds
of clouds will serve as an introduction to the
1— subject. The name stratus implies a horizontal
sheet of cloud having no special structure, except
that it is a thin layer. It was first applied
by Howard specifically to the thin layers that
form within a short distance of the earth’s
surface in quiet places on a still evening, preliminary
to the general formation of low fog at night; there¬
fore it is sometimes called high or lifted fog. It is
also applicable to thicker layers that form at con¬
siderable heights above the ground, and by cutting off
radiation effectually prevent the formation of fog at
the earth’s surface. Again, the term is applied to layers of
haze that may form even at the highest altitude. Hear
the observer’s zenith such a layer usually appears as haze,
but when seen in the distant horizon it appears as a hori¬
zontal layer of stratus. Finally the term is applied in
various combinations to indicate, for instance, that the main
cloud has a broad thin base, like a stratus, attached to its
lower part or broad horizontal sheets overflowing at the top.
S. VI. — 88