Nature

==Atmosphere, climate, and weather==
[[File:Top of Atmosphere.jpg|thumb|[[Rayleigh scattering|Blue light is scattered more]] than other wavelengths by the gases in the [[atmosphere]], giving the Earth a blue [[Halo (optical phenomenon)|halo]] when seen from space.]]
{{Main|Atmosphere of Earth|Climate|Weather}}
The Earth's atmosphere is a key factor in sustaining the [[Nature#Ecosystems|ecosystem]]. The thin layer of gases that envelops the Earth is held in place by gravity. Air is mostly [[nitrogen]], [[oxygen]], [[water vapor]], with much smaller amounts of carbon dioxide, argon, etc. The atmospheric pressure declines steadily with altitude. The [[ozone layer]] 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.<ref>{{Cite book|url=https://books.google.com/books?id=AJ4SnHbb-ZcC&pg=PA42|title=Environmental Science: Problems, Connections and Solutions|last1=Miller|last2=Spoolman|first2=Scott|date=September 28, 2007|publisher=Cengage Learning|isbn=978-0-495-38337-6}}</ref> [[Ocean current]]s 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 region]]s much colder.
[[File:Lightnings sequence 2 animation-wcag.gif|thumb|left|[[Lightning]]]]
Weather can have both beneficial and harmful effects. Extremes in weather, such as [[tornado]]es or [[hurricane]]s and [[cyclone]]s, can expend large amounts of energy along their paths, and produce devastation. Surface vegetation has evolved a dependence on the seasonal variation of the weather, and sudden changes lasting only a few years can have a dramatic effect, both on the vegetation and on the animals which depend on its growth for their food.

Climate is a measure of the long-term trends in the weather. Various factors are known to [[Climate variability and change|influence the climate]], including ocean currents, surface [[albedo]], [[greenhouse gas]]es, variations in the solar luminosity, and changes to the Earth's orbit. Based on historical and geological records, the Earth is known to have undergone drastic climate changes in the past, including [[ice age]]s.
[[File:Dszpics1.jpg|thumb|A tornado in central [[Oklahoma]]]]
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 Earth 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 limited to only a few days.<ref>{{Cite journal|last1=Stern|first1=Harvey|last2=Davidson|first2=Noel|date=May 25, 2015|title=Trends in the skill of weather prediction at lead times of 1–14 days|journal=Quarterly Journal of the Royal Meteorological Society|volume=141|issue=692|pages=2726–2736|doi=10.1002/qj.2559|bibcode=2015QJRMS.141.2726S|s2cid=119942734|doi-access=free}}</ref> Overall, two things are happening worldwide: (1) temperature is increasing on the average; and (2) regional climates have been undergoing noticeable changes.<ref>{{cite web|title=Tropical Ocean Warming Drives Recent Northern Hemisphere Climate Change|website=Science Daily|date=April 6, 2001|url=https://www.sciencedaily.com/releases/2001/04/010406073554.htm|access-date=May 24, 2006|archive-url=https://web.archive.org/web/20060421222313/http://www.sciencedaily.com/releases/2001/04/010406073554.htm|archive-date=April 21, 2006|url-status=live|df=mdy-all}}</ref>