Professor Langley determines quantitatively the effects respectively
produced by the radiations from the solar spots, penumbra, and
photosphere upon the face of a thermopile, and infers that these
effects measure their relative influence on terrestrial climate.
In thus assuming that the heat communicated to the thermopile measures
the solar contribution to terrestrial climate, Professor Langley omits
an important factor, viz., the amount of heat absorbed in traversing
the earth’s atmosphere; and in measuring the relative efficiency of
the spots, penumbra, and photosphere, he has not taken into account
the variations of diathermancy of the intervening atmospheric matter,
which are due to the variations in the source of heat.
Speaking generally, it may be affirmed that the radiations of obscure
heat are more largely absorbed by the gases and vapors of our
atmosphere than those of luminous heat, and the great differences in
the mere luminosity of the spots, penumbra, and photosphere justify
the assumption that the radiations of a sun-spot will (to use the
expressive simile of Tyndall) lose far more by atmospheric sifting than
will those from the photosphere.
But the spot areas will be none the less effective on terrestrial
climate on that account. A given amount of heat arrested by the earth’s
atmosphere will have even greater climatic efficiency than if received
upon its solid surface, inasmuch as the gases are worse radiators than
the rocks, and will therefore, _cæteris paribus_, retain a larger
proportion of the heat they receive.
I have long ago endeavored to show[9] that the depth of the
photosphere, from the solar surface inwards, is limited by
dissociation; that the materials of the Sun within the photosphere
exist in a dissociated, elementary condition; that at the photosphere
they are, for the most part, combined. This view has since been adopted
by many eminent solar physicists, and if correct, demands a much higher
temperature within the depths revealed by that withdrawal of the
photospheric veil which constitutes a sun-spot.
If I am right in this, and also in supposing the spot-radiations to be
so much more abundantly absorbed than those of the photosphere, and if
in spite of this higher temperature of the spots, the _surface_ of the
earth receives from them the lower degree of heat measured by Professor
Langley, another interesting consequence must follow. The excess of
spot-heat directly absorbed by the atmosphere, and mainly by the
water dissolved or suspended in its upper regions, must be especially
effective in dissipating clouds and checking or modifying their
formation. The meteorological results of this may be important, and are
worthy of careful study.
In thus venturing to question some of Professor Langley’s inferences I
am far from underrating the interest and importance of his researches.
On the contrary, I regard the quantitative results he has obtained
as especially valuable and opportune, in affording means of testing
the above-named and other speculations in solar physics. Similar
observations repeated at different elevations would decide, so far as
the lower regions are concerned, whether or not there is any difference
in the quantity of heat imparted by the bright and obscure portions
of the Sun to our atmosphere. If the differences already observed by
Professor Langley vary in ascending, a new means will be afforded of
studying the constitution of the interior of the Sun and its relations
to the photosphere. Direct evidence of selective absorption by our
atmosphere may thus be obtained, which would go far towards solving one
of the crucial solar problems, viz., whether the darker regions are
hotter or cooler than the photosphere.
The obscure radiations from the moon must be absorbed by our atmosphere
like those from the sun-spot, and may be sufficiently effective to
account for the alleged dissipation of clouds by the full moon.
In both cases the climatic influence is greatly heightened by the fact
that all the heat thus absorbed is directly effective in raising the
temperature of the air. The action of the absorbed heat in reference
to cloud-formation is directly opposite to that of the transmitted
solar heat, as this reaching the surface of the earth evaporates the
superficial water, and thereby produces the material of clouds. On
the other hand, the heat which is absorbed by the air increases its
vapor-holding capacity, and thus prevents the formation of clouds, or
even effects the dissolution of clouds already formed.