The absorption of electromagnetic radiation by water in the gas phase occurs in three regions of the spectrum. Rotational transitions are responsible for absorption in the microwave and far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur in the vacuum ultraviolet regions. Liquid water has no rotational spectrum but does absorb in the microwave region. Ice has a similar spectrum to liquid water.
The water molecule, in the gaseous state, has three types of transition that can give rise to absorption of electromagnetic radiation
Rotational transitions, in which the molecule gains a quantum of rotational energy. Atmospheric water vapour at ambient temperature and pressure gives rise to absorption in the far-infrared region of the spectrum, from about 200 cm−1 (50 μm) to longer wavelengths towards the microwave region.
Vibrational transitions in which a molecule gains a quantum of vibrational energy. The fundamental transitions give rise to absorption in the mid-infrared in the regions around 1650 cm−1 (μ band, 6 μm) and 3500 cm−1 (X-band, 2.9 μm)
Electronic transitions in which a molecule is promoted to an excited electronic state. The lowest energy transition of this type is in the vacuum ultraviolet region.
In liquid water the rotational transitions are effectively quenched, but absorption bands are affected by hydrogen bonding. In crystalline ice the vibrational spectrum is also affected by hydrogen bonding and there are lattice vibrations causing absorption in the far-infrared. Electronic transitions of gaseous molecules will show both vibrational and rotational fine structure.
Infrared absorption band positions may be given either in wavelength, micrometers, μm, often shortened to "microns", or wavenumbers per centimeter, cm−1, sometimes referred to as reciprocal centimeters. Since there are 104 micrometers in 1 centimeter, the two units are related by
wavenumber (cm-1) = 104 / wavelength (μm)
Wavenumber per centimeter is the reciprocal of the wavelength in cm.
Part of the pure rotation (microwave) spectrum of water vapour between 1000 GHz ( 33 cm-1, 300 μm) and 3000 GHz (100 cm-1, 100 μm)
The water molecule has three fundamental molecular vibrations. The O-H stretching vibrations give rise to absorption bands with band origins at 3657 cm−1 (ν1, 2.734 μm) and 3756 cm−1 (ν3, 2.662 μm) in the gas phase. The asymmetric stretching vibration, of B2 symmetry in the point group C2v is a normal vibration. The H-O-H bending mode origin is at 1595 cm−1 (ν2, 6.269 μm). Both symmetric stretching and bending vibrations have A1 symmetry, but the frequency difference between them is so large that mixing is effectively zero. In the gas phase all three bands show extensive rotational fine structure. ν3 has a series of overtones at wavenumbers somewhat less than n ν3, n=2,3,4,5... Combination bands, such as ν2 + ν3 are also easily observed in the near infrared region. The presence of water vapor in the atmosphere is important for atmospheric chemistry especially as the infrared and near infrared spectra are easy to observe. Standard (atmospheric optical) codes are assigned to absorption bands as follows. 0.718 μm (visible): α, 0.810 μm: μ, 0.935 μm: ρστ, 1.13 μm: φ, 1.38 μm: ψ, 1.88 μm: Ω, 2.68 μm: X. The gaps between the bands define the infrared window in the Earth's atmosphere.
The infrared spectrum of liquid water is dominated by the intense absorption due to the fundamental O-H stretching vibrations. Because of the high intensity, very short path lengths, usually less than 50 μm, are needed to record the spectra of aqueous solutions. There is no rotational fine structure, but the absorption band are broader than might be expected, because of hydrogen bonding. Peak maxima for liquid water are observed at 3450 cm−1 (2.898 μm), 3615 cm−1 (2.766 μm) and 1640 cm −1 (6.097 μm). Direct measurement of the infrared spectra of aqueous solutions requires that the cuvette windows be made of substances such as calcium fluoride which are water insoluble. This difficulty can be overcome by using an Attenuated total reflectance (ATR) device.
In the near-infrared range liquid water has absorption bands around 1950 nm (5128 cm−1), 1450 nm (6896 cm−1), 1200 nm (8333 cm−1) and 970 nm, (10300 cm−1). The regions between these bands can be used in near-infrared spectroscopy to measure the spectra of aqueous solutions, with the advantage that glass is transparent in this region, so glass cuvettes can be used. The absorption intensity is weaker than for the fundamental vibrations, but this is not important as longer path-length cuvettes are used. The absorption band at 698 nm (14300 cm−1) is a 3rd overtone (n=4). It tails off onto the visible region and is responsible for the intrinsic blue color of water. This can be observed with a standard UV/vis spectrophotometer, using a 10 cm path-length. The colour can be seen by eye by looking through a column of water about 10m in length; the water must be passed through an ultrafilter to eliminate color due to Rayleigh scattering which also can make water appear blue. In both liquid water and ice cluster vibrations occur, which involve the stretching (TS) or bending (TB) of intermolecular hydrogen bonds (O–H . . . O). Bands at wavelengths λ = 50-55 μm (44 μm in ice) have been attributed to TS, intermolecular stretch, and 200 μm (166 μm in ice), to TB, intermolecular bend
The spectrum of ice is similar to that of liquid water, with peak maxima at 3400 cm−1 (2.941 μm), 3220 −1 (3.105 μm) and 1620 −1 (6.17 μm)
Very weak light absorption, in the visible region, by liquid water has been measured using an integrating cavity absorption meter (ICAM). The absorption was attributed to a sequence of overtone and combination bands whose intensity decreases at each step, giving rise to an absolute minimum at 418 nm, at which wavelength the attenuation coefficient is about 0.0044 m−1, meaning that energy of 418 nm light drops to one thousandth after traveling about 1570 metres in water.
Predicted wavelengths of overtones and combination bands of liquid water in the visible region
The electronic transitions of the water molecule lie in the vacuum ultraviolet region. For water vapor the bands have been assigned as follows.
Dielectric permittivity and dielectric loss of water between 0°C and 100°C, the arrows showing the effect of increasing temperature.
The pure rotation spectrum of water vapor extends into the microwave region.
Liquid water has a broad absorption spectrum in the microwave region, which has been explained in terms of changes in the hydrogen bond network giving rise to a broad, featureless, microwave spectrum. The absorption (equivalent to dielectric loss) is used in microwave ovens to heat food that contains water molecules. A frequency of 2.45 GHz, wavelength 122 mm, is commonly used.
Radiocommunication at GHz frequencies is very difficult in fresh waters and even more so in salt waters. 
Synthetic stick absorption spectrum of a simple gas mixture corresponding to the Earth's atmosphere composition based on HITRAN data  created using Hitran on the Web system . Green color - water vapor, WN - wavenumber (caution: lower wavelengths on the right, higher on the left). Water vapor concentration for this gas mixture is 0.4%
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 greenhouse effect. 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 further enhances the Earth’s greenhouse effect.
As well as absorbing radiation, water vapour emits radiation in all directions. Much of this energy will be recaptured by other water molecules, but as it ascends there is less water above capable of recapturing radiation of water-specific wavelengths sent towards space. By the top of the troposphere, about 12 km above sea level, most water vapour will have already condensed to liquid or ice and released its heat of vapourisation. Liquid water and ice will fall to lower heights. Temperatures at this altitide known as the tropopause are about -50 degrees celsius.
In the atmospheric window between approximately 8000 and 14000 nm, in the far-infrared spectrum, carbon dioxide and water absorption is weak. This window allows most of the thermal radiation in this band to be radiated out to space directly from the Earth's surface. This band is also used for remote sensing of the Earth from space, for example with thermal Infrared imaging.
^ ab"Spectroscopy of Atmospheric Gases (spectral databases)". V.E. Zuev Institute of Atmospheric Optics SB RAS. Retrieved August 8, 2012. "... various data sources: HITRAN and GEISA spectral databanks, original data obtained by IAO researchers in collaboration with other scientists, H2O spectra simulated by Partridge and Schwenke etc... ..."
^ abc"The HITRAN Database". Atomic and Molecular Physics Division, Harvard-Smithsonian Center for Astrophysics. Retrieved August 8, 2012. "HITRAN is a compilation of spectroscopic parameters that a variety of computer codes use to predict and simulate the transmission and emission of light in the atmosphere."
^ abc"Hitran on the Web Information System". Harvard-Smithsonian Center for Astrophysics (CFA), Cambridge, MA, USA; V.E. Zuev Institute of Atmosperic Optics (IAO), Tomsk, Russia. Retrieved August 11, 2012.
^ abGordon, 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. 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 applications."Cite uses deprecated parameters (help)
^Banwell, Colin N.; McCash, Elaine M. (1994). Fundamentals of molecular spectroscopy (4th edition ed.). McGraw-Hill. p. 50. ISBN0-07-707976-0.
^ abcNakamoto, Kazuo (1997). Infrared and Raman spectra of inorganic and coordination compounds (5th edition ed.). Wiley. p. 170. ISBN0-47116394-5.
^Duarte, F. J., Edited (1995). Tunable Laser Applications. New York: M. Dekker. ISBN0-8247-8928-8. "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..."
^Chaplin, Martin (2007-10-28). "Water Absorption Spectrum". 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."
^Cotton, William (2006). Human Impacts on Weather and Climate. Cambridge: Cambridge University Press. ISBN0-521-84086-4. "Little absorption is evident in the region called the atmospheric window between 8 and 14 μm"