spectrum, absorption by carbon dioxide and water is weak. This window allows most of the thermal radiation of these wavelengths to be radiated out to space The absorption of electromagnetic radiation by water depends on the state of the water. The absorption 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 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 where it absorbs 60% of the atmospheric absorption of thermal radiation emitted by the Earth in a process known as the greenhouse effect. It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing because water vapor absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimeter wave bands. The South Pole Telescope was constructed in Antarctica in part because its high elevation and low temperatures mean there is very little water vapor 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% 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. In the atmospheric window between approximately 8000 and 14000 nm, in the far-infrared spectrum, absorption by carbon dioxide 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. In the near-infrared spectrum, 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, 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 where it absorbs 60% of the atmospheric absorption of thermal radiation emitted by the Earth in a process known as the greenhouse effect. It is also an important factor in multispectral imaging and hyperspectral imaging used in remote sensing because water vapor absorbs radiation differently in different spectral bands. Its effects are also an important consideration in infrared astronomy and radio astronomy in the microwave or millimeter wave bands. The South Pole Telescope was constructed in Antarctica in part because its high elevation and low temperatures mean there is very little water vapor 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% 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. In the atmospheric window between approximately 8000 and 14000 nm, in the far-infrared spectrum, absorption by carbon dioxide and water is weak. This window allows most of the thermal radiation of these wavelengths 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 with for example thermal Infrared imaging. As well as absorbing radiation, water vapour occasionally emits radiation in all directions, according to the black-body emission curve for its current temperature overlaid on the water absorption spectrum. Much of this energy will be recaptured by other water molecules, but at higher altitudes, radiation sent towards space is less likely to be recaptured since there is less water available to recapture radiation at water-specific absorbing wavelengths. By the top of the troposphere, about 12 km above sea level, most water vapor condenses to liquid water or ice as it releases its heat of vapourization. After changing phase, liquid water and ice fall away to lower altitudes. This loss will be balanced by incoming water vapour rising via convection currents. Liquid water and ice emit radiation at a higher rate than water vapour (see graph above). Water at the top of the troposphere, particularly in liquid and solid states, cools as it emits net photons to space. Neighboring gas molecules other than water (e.g. nitrogen) are cooled by passing their heat kinetically to the water. This is why temperatures at the top of the troposphere (known as the tropopause) are about −50 degrees Celsius.