Electromagnetic Absorption by Water
During the transmission of electromagnetic radiation through a medium containing water molecules, portions of the electromagnetic spectrum are absorbed by the water molecules. This water absorption occurs preferentially at certain characteristic wavelengths while the balance of the spectrum is transmitted with minimal effects.
Strong absorbance by water vapour occurs at wavelengths around 2900, 1950 and 1450 nanometers (nm), with weaker absorption around 1200 and 970 nm, and three additional sets of water-vapour 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 vapour ranging from the microwave region to the visible spectrum.
The spectral absorption features of liquid water are shifted to longer wavelengths with respect to the vapour features by approximately 60 nm. In hexagonal ice, the features are shifted even further into the red and infrared. In liquid water and ice the infrared and Raman spectra are far more complex than in the vapour.
Water vapour is a greenhouse gas in the Earth's atmosphere, responsible for 70 percent of the known absorption of incoming sunlight, particularly in the infrared region, and about 60 percent of the atmospheric absorption of thermal radiation by the Earth known as the greenhouse effect. It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing because water vapour absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimetre 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 vapour in the atmosphere.
Similarly, carbon dioxide absorption bands occur around 1400, 1600 and 2000 nm, but its presence in the Earth's atmosphere accounts for just 26 percent of the greenhouse effect. Carbon dioxide gas absorbs energy in some small segments of the thermal infrared spectrum that water vapour 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 vapour. This extra water vapour absorption further enhances the Earth's greenhouse effect.
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, keeping the Earth's atmosphere from going into thermal runaway. This band is also used for remote sensing of the Earth from space, for example with VNIR imaging.
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