THE "CHALLENGER" WEATHER REPORTS
The _Challenger_ Expedition, commenced by Sir Wyville Thomson, and after
his death continued by Sir John Murray, with an able staff of assistants
for the several departments, was one of the splendid exceptions to the
ordinary British Government stinginess in the furtherance of science. The
results of the Expedition were printed in a great number of very handsome
volumes at the expense of the Government.
And the valuable deductions from the _Challenger's_ Weather Reports by Dr.
Alex. Buchan, in his "Atmospheric Circulation," have thrown considerable
light upon oceanic weather phenomena. For some of his matured opinions on
these, I am here much indebted to him.
Humboldt, in 1817, published a treatise on "Isothermal Lines," which
initiated a fresh line for the study of atmospheric phenomena. An isotherm
is an imaginary line on the earth's surface, passing through places having
a corresponding temperature either throughout the year or at any
particular period. An isobar is an imaginary line on the earth's surface,
connecting places at which the mean height of the barometer at sea-level
is the same. To isobars, as well as to isotherms, Dr. Buchan has devoted
considerable attention. In 1868, he published an important series of
charts containing these, with arrows for prevailing winds over the earth
for the months of the year. In this way what are called synoptic charts
were established.
In the _Challenger_ Report are shown the various movements of the
atmosphere, with their corresponding causes. It is thus observed that the
prevailing winds are produced by the inequality of the mass of air at
different places. The air flows from a region of higher to a region of
lower pressure, _i.e._ from where there is an excessive mass of air to
fill up some deficiency. And this is the great principle on which the
science of meteorology rests, not only as to winds, but as to weather
changes.
Of the sun's rays which reach the earth, those that fall on the land are
absorbed by the surface layer of about 4 feet in thickness. But those that
fall on the surface of the ocean penetrate, as shown by the observations
of the _Challenger_ Expedition, to a depth of about 500 feet. Hence, in
deep waters the temperature of the surface is only partially heated by the
direct rays of the sun. In mid-ocean the temperature of the surface
scarcely differs 1 deg. Fahr. during the whole day, while the daily variation
of the surface layer of land is sometimes 50 deg. The temperature of the air
over the ocean is about three times greater than that of the surface of
the open sea over which it lies; but, near land, this increases to five
times.
The elastic force of vapour is seen in its simplest form on the open sea,
as disclosed by these Reports. It is lowest at 4 A.M. and highest at 2
P.M. The relative humidity is just the reverse. When the temperature is
highest, the saturation of the air is lowest, and _vice versa_. So on land
when the air, by radiation of heat from the earth, is cooled below the
dew-point, dew is produced, and, at the freezing-point, hoar-frost.
The _Challenger_ Reports, too, show that the force of the winds on the
open sea is subject to no distinct and uniform daily variation, but that
on nearing land the force of the wind gives a curve as distinctly marked
as the ordinary curve of temperature. That force is lowest from 2 to 4
A.M., and highest from 2 to 4 P.M. Each of the five great oceans gives the
same result. At Ben Nevis, on the other hand, these forces are just
reversed in strength.
It is also shown by the _Challenger_ observations that on the open sea the
greatest number of thunder-storms occur from 10 P.M. to 8 A.M. And, from
this, Dr. Buchan concludes that over the ocean terrestrial radiation is
more powerful than solar radiation in causing those vertical disturbances
in the equilibrium of the atmosphere which give rise to the thunder-storm.