Air & Tur

The atmosphere of Earth consists of a multi-layered gas mixture (commonly referred to as air) surrounding the Earth's planetary surface, which is retained by gravity. Like the atmospheres of other planets, Earth's atmosphere also contains variable quantities of suspended aerosols and particulates that create weather features such as clouds and hazes. The atmosphere serves as a protective buffer between the Earth's crust and the outer space, shielding the surface from most meteoroids and ultraviolet solar radiation. It also reduces diurnal temperature variation (i.e. the temperature extremes between day and night), keeps the Earth's surface warm through heat retention via the greenhouse effect, redistributes heat and moisture convectively among different regions via air currents, and provides the chemical and climate conditions that allow life to exist and evolve on Earth. By mole fraction (i.e., by quantity of molecules), dry air contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other trace gases (see Composition below for more detail). Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Earth's primordial atmosphere consisted of mainly hydrogen and helium gases accreted from the solar nebula, but the composition changed significantly over time, affected by many factors such as atmospheric escape (primarily of the more volatile hydrogen and helium), volcanism and outgassing, impact events and weathering. By the Archean eon, the Earth's atmosphere transitioned into a secondary atmosphere dominated by nitrogen and methane with other mixtures such as water vapor, carbon dioxide, hydrogen sulfide and sulfur dioxide. This prebiotic atmosphere, however, further changed with the evolution of life, particularly the rise of cyanobacteria — chlorophyll-bearing photoautotrophs capable of oxygenic photosynthesis, i.e. using (red and blue spectrum) sunlight to split water and fix carbon while releasing highly reactive dioxygen as the chemical byproduct. This continuous biotic oxygen release eventually depleted all reductants available on the Earth's surface, initially in the hydrosphere and later in the lithosphere and the atmosphere, permanently converting the once-reducing atmosphere into an oxidative atmosphere in a geological event at the end of the Archean known as the Great Oxygenation Event. During the subsequent Proterozoic and Phanerozoic eons, the Earth's atmosphere — which is actually its tertiary atmosphere — experienced several further rounds of oxygenation, resulting in the nitrogen/oxygen-dominated atmosphere of today. In the present day, human activity has contributed to atmospheric changes, such as climate change (mainly through deforestation and fossil-fuel–related global warming), ozone depletion and acid deposition. The atmosphere has a mass of about 5.15×1018 kg, three-quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line at 100 km (62 mi) is often used as a conventional definition of the edge of space. Several layers can be distinguished in the atmosphere based on characteristics such as temperature and composition, namely the troposphere, stratosphere, mesosphere, thermosphere (formally the ionosphere), and exosphere. Air composition, temperature and atmospheric pressure vary with altitude. Air suitable for use in photosynthesis by terrestrial plants and respiration of terrestrial animals is found within the troposphere. The study of Earth's atmosphere and its processes is called atmospheric science (aerology), and includes multiple subfields, such as climatology and atmospheric physics. Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann. The study of the historic atmosphere is called paleoclimatology.