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Solar spectrum above the atmosphere (blue curve) and transmitted
Earth's surface (orange curve) showing major absorption
Water absorption is a phenomenon in the
transmission of electromagnetic radiation
through a medium containing water molecules. Water molecules are excited by
radiation at certain wavelengths and tend to selectively absorb portions of the spectrum while allowing the
balance of the spectrum to be transmitted with minimal effect.
vapor absorption bands
occur at wavelengths
around 2500, 1950 and 1450 nanometers (nm),
with weaker absorption around 1200 and 970 nm,
and three additional sets of water-vapor absorption lines near 930,
820, and 730 nm, all in
the infrared spectrum.
Water has a complex absorption spectrum — the 2007 HITRAN spectroscopy database update lists more
than 64,000 spectral
lines corresponding to significant transitions of water vapor
ranging from the microwave region to the visible
The liquid water absorption features are offset to longer
wavelengths from the water vapor absorption features by about 60
nm. In hexagonal ice, the features are shifted
even further. In liquid water and ice the infrared and Raman
spectra are far more complex than in the vapor.
Water vapor is a greenhouse gas in the Earth's atmosphere, responsible for 70% of
the known absorption of incoming sunlight, particularly in the infrared region,
and about 60% of the atmospheric absorption of thermal
radiation by the Earth known as the greenhouse
It is also an important factor in multispectral
imaging and hyperspectral imaging used in remote
because water vapor absorbs radiation differently in different
spectral bands. Its effects are also an important consideration in
astronomy and radio astronomy in the microwave or millimeter wave bands. The South
Pole Telescope was constructed in Antarctica in part because the elevation and
low temperatures there mean there is very little water vapor in the
Similarly, carbon dioxide absorption bands occur
around 1400, 1600 and 2000 nm, but
its presence in the Earth's atmosphere accounts for just 26% of the
Carbon dioxide gas absorbs energy in some small segments of the
thermal infrared spectrum that water vapor misses. This extra
absorption within the atmosphere causes the air to warm just a bit
more and the warmer the atmosphere the greater its capacity to hold
more water vapor. This extra water vapor absorption then further
enhances the Earth’s greenhouse effect.
Conversely, there is an atmospheric window between
approximately 800 and 1400 nm, in the near-infrared spectrum
where carbon dioxide and water absorption is weak. This
window allows most of the thermal radiation in this band to be
radiated out to space, keeping the Earth's atmosphere from going
runaway. This band is also used for remote sensing of the Earth
from space, for example with VNIR
The water vapor absorption bands are related to molecular
vibrations involving different combinations of the water molecule's three fundamental
- V1: symmetric stretch mode
- V2: bending mode
- V3: asymmetric stretch mode
The absorption feature centered near 970 nm is attributed to a
2V1 + V3 combination, the one near 1200 nm to a V1 + V2 + V3
combination, the one near 1450 nm to a V1 + V3 combination, and the
one near 1950 nm to a V2 + V3 combination.
In liquid water, rotations tend to be restricted by hydrogen bonds, leading to librations, or
rocking motions. Also stretching is shifted to a lower frequency
while the bending frequency increased by hydrogen bonding.
|Three fundamental vibrations of the water molecule
- ^ Carter, G.A.; McCain, D.C. (1993). "Relationship of leaf
spectral reflectance to chloroplast water content determined using
NMR microscopy". Remote Sensing of Environment
46 (3): 305–310. doi:10.1016/0034-4257(93)90050-8. http://www.csa.com/partners/viewrecord.php?requester=gs&collection=ENV&recid=3615755. Retrieved 2007-10-31.
"Reflectance responses to leaf water content were greatest in the
water absorption bands near 1450 nm, 1950 nm, and 2500 nm
- ^ Rossel, R.A.V.; McBratney, A.B. (1998). "Laboratory evaluation of a
proximal sensing technique for simultaneous measurement of soil
clay and water content". Geoderma 85
(1): 19–39. doi:10.1016/S0016-7061(98)00023-8. http://linkinghub.elsevier.com/retrieve/pii/S0016706198000238. Retrieved 2007-10-31. "the
strong absorption bands of OH groups in soil water at around 1450,
1950 and 2500 nm.".
- ^ a
Jacquemoud, S.; Ustin, S.L. (2003).
"Application of radiative
transfer models to moisture content estimation and burned land
mapping". Joint European Association of Remote Sensing
Laboratories (EARSeL) and GOFC/GOLD-Fire Program, 4th Workshop on
Forest Fires, University Ghent, Belgium 5--7 June 2003. http://www.ipgp.jussieu.fr/~jacquemoud/publications/jacquemoud2003.pdf. Retrieved 2008-10-15. "...in
the action spectrum of water the three main peaks near 1400, 1950,
and 2500 nm, and two minor ones at 970 and 1200 nm".
Duarte, Edited (1995). Tunable
Laser Applications. New York: M. Dekker. ISBN 0824789288.
"There are three sets of water-vapor absorption lines in the
near-IR spectral region. Those near 730 and 820 nm are useful for
lower tropo- spheric measurements, whereas those near 930 nm are
useful for upper- tropospheric measurements..."
- ^ a
Gordon, Iouli E.; Laurence S. Rothman,
Robert R. Gamache, David Jacquemart, Chris Boone, Peter F.
Bernathd, Mark W. Shephard, Jennifer S. Delamere, Shepard A. Clough
(2007-06-24). "Current updates of the
water-vapor line list in HITRAN: A new ‘‘Diet’’ for air-broadened
half-widths" (pdf). Journal of Quantitative Spectroscopy
& Radiative Transfer. http://www.cfa.harvard.edu/atmosphere/publications/2007-Diet-JQSRT-XXX.pdf. Retrieved 2007-11-03. "Water
vapor is the principal absorber of longwave radiation in the
terrestrial atmosphere and it has a profound effect on the
atmospheric energy budget in many spectral regions. The HITRAN
database lists more than 64,000 significant transitions of water
vapor ranging from the microwave region to the visible, with
intensities that cover many orders of magnitude. These transitions
are used, or have to be accounted for, in various remote-sensing
Toselli, F. (1992). Imaging
Spectroscopy. Boston: Kluwer Academic Publishers. ISBN 0792315359.
"The liquid water absorption features are offset to longer
wavelengths from the water vapor absorption features by about 60
- ^ a
Chaplin, Martin (2007-10-28). "Water Absorption
Spectrum". http://www.lsbu.ac.uk/water/vibrat.html. Retrieved 2007-11-04. "In
the liquid, rotations tend to be restricted by hydrogen bonds,
giving the librations. Also, spectral lines are broader causing
overlap of many of the absorption peaks. The main stretching band
in liquid water is shifted to a lower frequency and the bending
frequency increased by hydrogen bonding."
- ^ a
Maurellis, Ahilleas (2003-05-01). "The climatic effects of water
vapour - physicsworld.com". Physics World. Institute
of Physics. http://physicsworld.com/cws/article/print/17402. Retrieved
"South Pole Telescope: South
Pole : Why is the telescope at the South Pole?". University of Chicago. http://spt.uchicago.edu/public/southpole.html. Retrieved 2007-11-03. "Quick
Answer: Because the South Pole is probably the best place on Earth
for this telescope. It is extremely dry, making the atmosphere
exceptionally transparent for SPT."
Prieto-Blanco, Ana; Peter R. J. North ,
Nigel Fox , Michael J. Barnsley. "Satellite estimation of
surface/atmosphere parameters: a sensitivity study" (pdf). http://geography.swan.ac.uk/personal/prjn/papers/Prietoetal2005.pdf. Retrieved 2007-10-31.
"...water absorption bands (around 940nm, 1100nm, 1450nm, 1950nm
and 2500nm) and carbon dioxide absorption bands (1400nm, 1600nm and
"EO Study: Does the Earth have
an Iris Analog". NASA. http://earthobservatory.nasa.gov/Study/Iris/. Retrieved
Cotton, William (2006). Human
Impacts on Weather and Climate. Cambridge: Cambridge
University Press. ISBN 0521840864.
"Little absorption is evident in the region called the atmospheric
window between 8 and 14 μm"