water vapor

A colorless trace gas that is part of the air and that plays an important role in atmospheric dynamics and the Earth's energy balance. It is incorporated into the atmosphere mainly through the evaporation of the large bodies of water covering the Earth's surface, with contributions from vegetation (evapotranspiration) and, to a lesser extent, from the snow and ice cover (sublimation), also being notable. It is the atmospheric component displaying the most irregular distribution, both spatially and temporally. Its proportion in the air can vary from barely 0.4% by volume in desert areas, to 4% in cloud forests and tropical rainforests, decreasing continuously as one ascends through the atmosphere. It is concentrated mainly in the lower and middle troposphere. It has a great capacity to absorb radiation coming from the sun (short wave) and from the Earth's surface (long wave), which makes it a prominent greenhouse gas and the main regulator of temperature at a planetary scale. At a popular level, there is considerable confusion regarding its nature, as the idea is widespread that clouds, the steaming of hot water, or the breath we exhale through our mouths on cold winter days, are all water vapor. In all those cases, what is observed is the result of a change of phase from vapor to liquid, since all those elements are made up of microscopic-sized water droplets, which arose from the condensation of the water vapor present in the air. Of the total water contained in the atmosphere - in any of its three states -, 96% is water vapor, although the average lifetime of a molecule of this gas in the atmospheric medium is only ten days.