Editing Weather (section)

== Causes ==
[[File:Stormclouds.jpg|thumb|right|upright=1.13|[[Cumulonimbus cloud]] surrounded by [[stratocumulus]]]]
On [[Earth]], common [[List of meteorological phenomena|weather phenomena]] include wind, [[cloud]], rain, snow, [[fog]] and [[dust storm]]s. Some more common events include [[natural disaster]]s such as [[tornado]]es, [[Tropical cyclone|hurricanes]], [[typhoons]] and [[Ice storm|ice storms]]. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere).<ref name=":0" /> Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.<ref>{{cite web|url=http://www.gsfc.nasa.gov/topstory/20011018windsurface.html |title=Weather Forecasters May Look Sky-high For Answers |last=O'Carroll |first=Cynthia M. |publisher=[[Goddard Space Flight Center]] (NASA) |date=18 October 2001 |url-status=dead |archive-url=https://web.archive.org/web/20090712090309/http://www.gsfc.nasa.gov/topstory/20011018windsurface.html |archive-date=12 July 2009 }}</ref>

Weather occurs primarily due to air pressure, temperature and [[moisture]] differences from one place to another. These differences can occur due to the [[sun]] angle at any particular spot, which varies by latitude in the tropics. In other words, the farther from the tropics one lies, the lower the sun angle is, which causes those locations to be cooler due to the spread of the [[sunlight]] over a greater surface.<ref>[[NASA]]. [https://web.archive.org/web/20050501070321/http://www.nasa.gov/worldbook/weather_worldbook.html World Book at NASA: Weather.] [https://web.archive.org/web/20101213184908/http://www.nasa.gov/worldbook/weather_worldbook.html Archived copy] at [[WebCite]] (10 March 2013). Retrieved on 27 June 2008.</ref> The strong temperature contrast between [[Polar region|polar]] and tropical air gives rise to the large scale [[atmospheric circulation]] cells and the [[jet stream]].<ref name="Stimac">John P. Stimac. [http://www.ux1.eiu.edu/~cfjps/1400/pressure_wind.html] {{Webarchive|url=https://web.archive.org/web/20070927210111/http://www.ux1.eiu.edu/~cfjps/1400/pressure_wind.html|date=27 September 2007}} [[Air pressure]] and wind. Retrieved on 8 May 2008.</ref> Weather [[systems]] in the mid-latitudes, such as [[extratropical cyclone]]s, are caused by instabilities of the [[jet stream]] flow (see [[baroclinity]]).<ref>Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss. [https://archive.today/20121208181523/http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175/1520-0493(1990)118%3C0234:ARCEDG%3E2.0.CO;2 A Rapid Cyclogenesis Event during GALE IOP 9.] Retrieved on 28 June 2008.</ref> Weather systems in the tropics, such as [[monsoon]]s or organized [[thunderstorm]] systems, are caused by different processes.
[[File:16-008-NASA-2015RecordWarmGlobalYearSince1880-20160120.png|thumb|left|upright=1.36|{{center|2015 – [[Global warming|5th Warmest Global Year]] on Record (since 1880) as of 2021 – Colors indicate temperature anomalies ([[NASA]]/[[NOAA]]; 20 January 2016).<ref name="NASA-20160120">{{cite web |last1=Brown |first1=Dwayne |last2=Cabbage |first2=Michael |last3=McCarthy |first3=Leslie |last4=Norton |first4=Karen |title=NASA, NOAA Analyses Reveal Record-Shattering Global Warm Temperatures in 2015 |url=http://www.nasa.gov/press-release/nasa-noaa-analyses-reveal-record-shattering-global-warm-temperatures-in-2015 |date=20 January 2016 |work=[[NASA]] |access-date=21 January 2016 |archive-date=20 January 2016 |archive-url=https://web.archive.org/web/20160120183259/http://www.nasa.gov/press-release/nasa-noaa-analyses-reveal-record-shattering-global-warm-temperatures-in-2015 |url-status=live }}</ref>}}]]
Because the Earth's [[Earth's rotation|axis]] is tilted relative to its orbital plane, [[sunlight]] is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the [[Sun]], so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see [[Effect of sun angle on climate]]).<ref>Windows to the Universe. [http://www.windows.ucar.edu/tour/link=/earth/climate/cli_seasons.html Earth's Tilt Is the Reason for the Seasons!] {{Webarchive|url=https://web.archive.org/web/20070808022809/http://www.windows.ucar.edu/tour/link=/earth/climate/cli_seasons.html |date=8 August 2007 }} Retrieved on 28 June 2008.</ref> This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of [[solar energy]] received by the [[Earth]] and influence long-term climate. (See [[Milankovitch cycles]]).<ref>Milankovitch, Milutin.  Canon of Insolation and the Ice Age Problem.  Zavod za Udz̆benike i Nastavna Sredstva: Belgrade, 1941. {{ISBN|86-17-06619-9}}.</ref>

The uneven solar heating (the formation of zones of temperature and moisture gradients, or [[frontogenesis]]) can also be due to the weather itself in the form of cloudiness and precipitation.<ref>Ron W. Przybylinski. [http://www.crh.noaa.gov/lsx/science/pdfppt/ron.ppt The Concept of Frontogenesis and its Application to Winter Weather Forecasting.] {{Webarchive|url=https://web.archive.org/web/20131024133948/http://www.crh.noaa.gov/lsx/science/pdfppt/ron.ppt |date=24 October 2013 }} Retrieved on 28 June 2008.</ref> Higher altitudes are typically cooler than lower altitudes, which is the result of higher surface temperature and radiational heating, which produces the [[Adiabatic process|adiabatic]] [[lapse rate]].<ref>{{cite book|author=Mark Zachary Jacobson|title=Fundamentals of Atmospheric Modeling|publisher=Cambridge University Press|edition=2nd|date=2005|isbn=978-0-521-83970-9|oclc=243560910}}</ref><ref>{{cite book|author=C. Donald Ahrens|title=Meteorology Today|publisher=Brooks/Cole Publishing|edition=8th|date=2006|isbn=978-0-495-01162-0|oclc=224863929}}</ref> In some situations, the temperature actually increases with height. This phenomenon is known as an [[Inversion (meteorology)|inversion]] and can cause mountaintops to be warmer than the valleys below. Inversions can lead to the formation of [[fog]] and often act as a [[Capping inversion|cap]] that [[Convective inhibition|suppresses]] thunderstorm development. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, [[ice]] sheets, or human-made objects) have differing physical characteristics such as [[albedo|reflectivity]], roughness, or moisture content.

Surface temperature differences in turn cause pressure differences. A hot surface warms the air above it causing it to expand and lower the density and the resulting surface [[Atmospheric pressure|air pressure]].<ref>Michel Moncuquet. [http://www.lesia.obspm.fr/~moncuque/theseweb/tempioweb/node6.html  Relation between density and temperature.] {{Webarchive|url=https://web.archive.org/web/20221127023536/https://www.lesia.obspm.fr/~moncuque/theseweb/tempioweb/node6.html |date=27 November 2022 }} Retrieved on 28 June 2008.</ref> The resulting horizontal [[Pressure gradient (atmospheric)|pressure gradient]] moves the air from higher to lower pressure regions, creating a wind, and the Earth's rotation then causes deflection of this airflow due to the [[Coriolis effect]].<ref>Encyclopedia of Earth. [http://www.eoearth.org/article/Wind Wind.] {{Webarchive|url=https://web.archive.org/web/20130509073805/http://www.eoearth.org/article/Wind |date=9 May 2013 }} Retrieved on 28 June 2008.</ref> The simple systems thus formed can then display [[Emergence|emergent behaviour]] to produce more [[Pressure system|complex systems]] and thus other weather phenomena.  Large scale examples include the [[Hadley cell]] while a smaller scale example would be [[sea breeze|coastal breezes]].

The [[Earth's atmosphere|atmosphere]] is a [[chaos theory|chaotic system]]. As a result, small changes to one part of the system can accumulate and magnify to cause large effects on the system as a whole.<ref>Spencer Weart. [http://www.aip.org/history/climate/chaos.htm The Discovery of Global Warming.] {{Webarchive|url=https://web.archive.org/web/20110607091743/http://www.aip.org/history/climate/chaos.htm |date=7 June 2011 }} Retrieved on 28 June 2008.</ref> This atmospheric instability makes weather forecasting less predictable than tidal waves or eclipses.<ref name="Lorenz, 1969">{{cite web|last1=Lorenz|first1=Edward|title=How Much Better Can Weather Prediction Become?|url=http://eaps4.mit.edu/research/Lorenz/How_Much_Better_Can_Weather_Prediction_1969.pdf|website=web.mit.edu/|publisher=Massachusetts Institute of Technology|access-date=21 July 2017|date=July 1969|archive-date=17 April 2016|archive-url=https://web.archive.org/web/20160417061111/http://eaps4.mit.edu/research/Lorenz/How_Much_Better_Can_Weather_Prediction_1969.pdf|url-status=dead}}</ref> Although it is difficult to accurately predict weather more than a few days in advance, [[weather forecasting|weather forecasters]] are continually working to extend this limit through [[meteorology|meteorological]] research and refining current methodologies in weather prediction. However, it is theoretically impossible to make useful day-to-day [[predictions]] more than about two weeks ahead, imposing an upper limit to [[potential]] for improved prediction skill.<ref name="Chaos in the Atmosphere">{{cite web|title=The Discovery of Global Warming: Chaos in the Atmosphere|url=https://history.aip.org/climate/chaos.htm|website=history.aip.org|access-date=21 July 2017|date=January 2017|archive-date=28 November 2016|archive-url=https://web.archive.org/web/20161128121540/https://history.aip.org/climate/chaos.htm|url-status=live}}</ref>