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===Atmosphere, climate and weather===
===Atmosphere, climate and weather===
The atmosphere of the Earth serves as a key factor in sustaining the planetary [[Nature#Ecosystems|ecosystem]]. The thin layer of [[gas]]es that envelops the Earth is held in place by the planet's gravity. Dry [[air]] consists of 78% [[nitrogen]], 21% [[oxygen]], 1% [[argon]] and other [[inert gases]], carbon dioxide, etc.; but air also contains a variable amount of [[water vapor]]. The atmospheric pressure declines steadily with altitude, and has a [[scale height]] of about 8 [[kilometre]]s at the Earth's surface: the height at which the atmospheric pressure has declined by a factor of ''[[E (mathematical constant)|e]]'' (a mathematical constant
The atmosphere of the Earth serves as a key factor in sustaining the planetary [[Nature#Ecosystems|ecosystem]]. The thin layer of [[gas]]es that envelops the Earth is held in place by the planet's gravity. Dry [[air]] consists of 78% [[nitrogen]], 21% [[oxygen]], 1% [[argon]] and other [[inert gases]], carbon dioxide, etc.; but air also contains a variable amount of [[water vapor]]. The atmospheric pressure declines steadily with altitude, and has a [[scale height]] of about 8 [[kilometre]]s at the Earth's surface: the height at which the atmospheric pressure has declined by a factor of ''[[E (mathematical constant)|e]]'' (a mathematical constant
equal to 2.71...).<ref>{{cite web
equal to 2.71...).[http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/Mathematical_Thinking/ideal_gases_under_constant.htm] [http://www.pnas.org/cgi/content/full/99/suppl_1/2546] The [[ozone layer]] of the Earth's atmosphere plays an important role in depleting the amount of [[ultraviolet]] (UV) radiation that reaches the surface. As [[DNA]] is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes.
Terrestrial weather occurs almost exclusively in the [[troposphere|lower part of the atmosphere]], and serves as a convective system for redistributing heat. Ocean currents are another important factor in determining climate, particularly the major underwater [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.
Terrestrial weather occurs almost exclusively in the [[troposphere|lower part of the atmosphere]], and serves as a convective system for redistributing heat. Ocean currents are another important factor in determining climate, particularly the major underwater [[thermohaline circulation]] which distributes heat energy from the equatorial oceans to the polar regions. These currents help to moderate the differences in temperature between winter and summer in the temperate zones. Also, without the redistributions of heat energy by the ocean currents and atmosphere, the tropics would be much hotter, and the polar regions much colder.
The climate of a region depends on a number of factors, especially [[latitude]]. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the [[tropical climate]] at the equator to the [[polar climate]] in the northern and southern extremes. Weather is also influenced by the [[seasons]], which result from the [[Earth]]'s [[axis of rotation|axis]] being [[axial tilt|tilted]] relative to its [[orbital plane (astronomy)|orbital plane]]. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the [[sun]]. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the [[northern hemisphere|northern]] and [[southern hemisphere|southern]] hemispheres experience opposite seasons.
The climate of a region depends on a number of factors, especially [[latitude]]. A latitudinal band of the surface with similar climatic attributes forms a climate region. There are a number of such regions, ranging from the [[tropical climate]] at the equator to the [[polar climate]] in the northern and southern extremes. Weather is also influenced by the [[seasons]], which result from the [[Earth]]'s [[axis of rotation|axis]] being [[axial tilt|tilted]] relative to its [[orbital plane (astronomy)|orbital plane]]. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the [[sun]]. This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the [[northern hemisphere|northern]] and [[southern hemisphere|southern]] hemispheres experience opposite seasons.
Weather is a [[Chaos theory|chaotic system]] that is readily modified by small changes to the [[Natural environment|environment]], so accurate [[Meteorology|weather forecasting]] is currently limited to only a few days. Overall, two things are currently happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.<ref>{{cite news | title=Tropical Ocean Warming Drives Recent Northern Hemisphere Climate Change | publisher=Science Daily|date=[[April 6]], [[2001]] | url=http://www.sciencedaily.com/releases/2001/04/010406073554.htm | accessdate=2006-05-24 }}</ref>
Weather is a [[Chaos theory|chaotic system]] that is readily modified by small changes to the [[Natural environment|environment]], so accurate [[Meteorology|weather forecasting]] is currently limited to only a few days. Overall, two things are currently happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.[http://www.sciencedaily.com/releases/2001/04/010406073554.htm]
== Life ==
== Life ==
