Geoscience Reference
In-Depth Information
Above the Vapor Screen
The Sun's rays shine not only in the visible part of the spectrum but over a range
from infrared to ultraviolet. Since the atmosphere absorbs infrared radiation rising
from the Earth's surface, as Pouillet noted, it must also absorb infrared rays en-
tering the atmosphere from the Sun. To understand the Earth's heat budget, it is
necessary to know how much heat the Sun provides before its rays begin to be af-
fected by the atmosphere. Tyndall had been delighted to learn that an American
scientist had emulated de Saussure by carrying to the mountaintop an instrument
to measure that quantity, known as the solar constant.
The scientist was Samuel Pierpont Langley (1824-1906), a Harvard graduate
who in 1867 had become director of the Allegheny Observatory in Pittsburgh.
1
Langley set up a research station on California's Mt. Whitney, at 14,505 feet the
highest peak in the continental United States. There, as he wrote to Tyndall, “the
air is perhaps drier than at any other equal altitude ever used for scientific invest-
neededamoresensitiveinstrumentthanthenexisted.LikeTyndall,heinventedhis
own, naming it the “bolometer” after the Greek word
bole
, for an object thrown, as
a ray of light. Langley's bolometer was said to be so sensitive that it could detect
the heat radiation emanating from a cow grazing a quarter-mile away.
Langley measured the solar constant and detected parts of the Sun's spectrum
never before seen. He confirmed that water vapor in the atmosphere does indeed
absorb much of the incoming infrared radiation. More than a century later, two cli-
mate scientists would write that “Langley's data are some of the best radiometric
observations ever under taken from the surface.”
3
100,000 Calculations
The Moon would surely seem the least likely place to learn about infrared absorp-
tion. After all, our satellite has no atmosphere to absorb anything. Generating no
light of its own, the Moon can but weakly reflect the light of the Sun. Or so it
seems. But by the mid-1800s scientists had discovered that the Moon does emit
infrared radiation. Using his bolometer, Langley measured the amount and from it
calculated the Moon's temperature.