R. W. Wood's "Note on the Theory of the Greenhouse"



The following sanctimonious statement appears in the current version
of the Wikipedia monograph on "Greenhouse Effect":

"The term "greenhouse effect" is a source of confusion in that actual
greenhouses do not warm by this mechanism (see section Real
greenhouses). Popular discussions often imply incorrectly that they
do; this error is sometimes made even in materials from scientific or
governmental agencies (e.g., the U.S. Environmental Protection
Agency[3])."

I find this extremely amusing because until recently this very same
Wikipedia monograph maintained -- not unsurprisingly -- that the
"greenhouse effect" was the mechanism by which greenhouses were
warmed. It was, in fact, because of this widely held belief that the
term "greenhouse effect" was created.

It has been hammered into our heads for decades that glass greenhouses
are heated by the greenhouse effect, which effect also accounts for
the heating of the atmosphere by greenhouse gases. The Wikipedia
monograph was recently forced to make an abrupt, and hopefully
embarrassed correction because of a century-old report of an
experiment (conducted by physicist R. W. Wood) that discredited the
original greenhouse effect.

The following article was published in the Philosophical Magazine in
1909 (Vol. 17, pp. 319-320):

XXIV. Note on the Theory of the Greenhouse

By Professor R. W. Wood (Communicated by the Author)

THERE appears to be a widespread belief that the comparatively high
temperature produced within a closed space covered with glass, and
exposed to solar radiation, results from a transformation of wave-
length, that is, that the heat waves from the sun, which are able to
penetrate the glass, fall upon the walls of the enclosure and raise
its temperature: the heat energy is re-emitted by the walls in the
form of much longer waves, which are unable to penetrate the glass,
the greenhouse acting as a radiation trap.

I have always felt some doubt as to whether this action played any
very large part in the elevation of temperature. It appeared much more
probable that the part played by the glass was the prevention of the
escape of the warm air heated by the ground within the enclosure. If
we open the doors of a greenhouse on a cold and windy day, the
trapping of radiation appears to lose much of its efficacy. As a
matter of fact I am of the opinion that a greenhouse made of a glass
transparent to waves of every possible length would show a temperature
nearly, if not quite, as high as that observed in a glass house. The
transparent screen allows the solar radiation to warm the ground, and
the ground in turn warms the air, but only the limited amount within
the enclosure. In the "open," the ground is continually brought into
contact with cold air by convection currents.

To test the matter I constructed two enclosures of dead black
cardboard, one covered with a glass plate, the other with a plate of
rock-salt of equal thickness. The bulb of a thermometer was inserted
in each enclosure and the whole packed in cotton, with the exception
of the transparent plates which were exposed. When exposed to sunlight
the temperature rose gradually to 65 oC., the enclosure covered with
the salt plate keeping a little ahead of the other, owing to the fact
that it transmitted the longer waves from the sun, which were stopped
by the glass. In order to eliminate this action the sunlight was first
passed through a glass plate.

There was now scarcely a difference of one degree between the
temperatures of the two enclosures. The maximum temperature reached
was about 55 oC. From what we know about the distribution of energy in
the spectrum of the radiation emitted by a body at 55 o, it is clear
that the rock-salt plate is capable of transmitting practically all of
it, while the glass plate stops it entirely. This shows us that the
loss of temperature of the ground by radiation is very small in
comparison to the loss by convection, in other words that we gain very
little from the circumstance that the radiation is trapped.

Is it therefore necessary to pay attention to trapped radiation in
deducing the temperature of a planet as affected by its atmosphere?
The solar rays penetrate the atmosphere, warm the ground which in turn
warms the atmosphere by contact and by convection currents. The heat
received is thus stored up in the atmosphere, remaining there on
account of the very low radiating power of a gas. It seems to me very
doubtful if the atmosphere is warmed to any great extent by absorbing
the radiation from the ground, even under the most favourable
conditions.

I do not pretend to have gone very deeply into the matter, and publish
this note merely to draw attention to the fact that trapped radiation
appears to play but a very small part in the actual cases with which
we are familiar.

gpl
.