View source for Synoptic scale meteorology

{{Short description|1000-km-order method of measuring weather systems}}
{{Atmospheric sciences}}
{{technical|date=October 2023}}

In [[meteorology]], the '''synoptic scale''' (also called the '''large scale''' or '''cyclonic scale''') is a [[horizontal position representation|horizontal]] [[length scale]] of the [[orders of magnitude (length)|order]] of {{cvt|1000|km|mi}} or more.<ref>{{cite web |title=cyclonic scale |url=https://glossary.ametsoc.org/wiki/cyclonic_scale |url-status=live |archive-url=https://web.archive.org/web/20160428104004/http://glossary.ametsoc.org/wiki/Cyclonic_scale |archive-date=28 April 2016 |access-date=2017-01-21 |publisher=[[American Meteorological Society]]}}</ref> This corresponds to a horizontal scale typical of [[mid-latitude]] [[depression (meteorology)|depressions]] (e.g. [[extratropical cyclone]]s). Most [[high-pressure area|high-]] and [[low-pressure area]]s seen on [[weather map]]s (such as [[surface weather analysis|surface weather analyses]]) are synoptic-scale systems, driven by the location of [[Rossby wave]]s in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while [[high-pressure area]]s form on the back edge of the trough. Most [[precipitation]] areas occur near frontal zones. The word ''[[wikt:synoptic|synoptic]]'' is derived from the [[Ancient Greek]] word {{Wikt-lang|grc|συνοπτικός}} ({{grc-transl|συνοπτικός}}), meaning "seen together".

The [[Navier–Stokes equations]] applied to atmospheric motion can be simplified by [[scale analysis (mathematics)|scale analysis]] in the synoptic scale. It can be shown that the main terms in horizontal equations are [[Coriolis force]] and [[pressure gradient]] terms; therefore, one can use [[geostrophic wind|geostrophic approximation]]. In vertical coordinates, the momentum equation simplifies to the [[hydrostatic equilibrium]] equation.

==Surface weather analysis==
[[Image:Surface analysis.gif|thumb|250px|A surface weather analysis for the United States on October 21, 2006]]
{{Main|Surface weather analysis}}
A [[surface weather analysis]] is a special type of [[weather map]] that provides a view of [[weather]] elements over a geographical area at a specified time based on information from ground-based weather stations.<ref>''Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather.''  University of Chicago PressChicago: 1999.</ref> Weather maps are created by plotting or tracing the values of relevant quantities such as [[sea level pressure]], [[temperature]], and [[cloud cover]] onto a [[geographical map]] to help find [[synoptic scale]] features such as [[weather fronts]].

The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems.<ref name="MILLER">Eric R. Miller. [http://docs.lib.noaa.gov/rescue/mwr/061/mwr-061-07-0189.pdf American Pioneers in Meteorology.] Retrieved on 2007-04-18.</ref> After the advent of the [[telegraph]], simultaneous [[surface weather observation]]s became possible for the first time. Beginning in the late 1840s, the [[Smithsonian Institution]] became the first organization to draw real-time surface analyses. Use of surface analyses began first in the United States, spreading worldwide during the 1870s. Use of the [[Norwegian cyclone model]] for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during [[World War II]].

Surface weather analyses have special symbols which show frontal systems, cloud cover, [[precipitation]], or other important information. For example, an ''H'' represents [[high-pressure area|high pressure]], implying good and fair weather. An ''L'' represents [[low pressure]], which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. Areas of precipitation help determine the frontal type and location. Mesoscale systems and boundaries such as [[tropical cyclone]]s, outflow boundaries and [[squall line]]s are also analyzed on surface weather analyses. Isobars are commonly used to place surface boundaries from the [[horse latitudes]] poleward, while streamline analyses are used in the tropics.<ref>Bureau of Meteorology. [http://www.bom.gov.au/nmoc/MSL/WeatherMap.shtml#types The Weather Map.] Retrieved on 10 May 2007.</ref>

==Extratropical cyclone==

[[File:UK-Cyclone.gif|thumb|250px|A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between [[isobar (meteorology)|isobars]] indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm [[Surface weather analysis|frontal boundaries]].]]

{{main|Extratropical cyclone}}

An extratropical cyclone is a synoptic scale [[Low-pressure area|low-pressure]] weather system that has neither [[tropical cyclone|tropical]] nor [[polar cyclone|polar]] characteristics, being connected with [[Surface weather analysis|fronts]] and horizontal [[gradients]] in [[temperature]] and [[dew point]] otherwise known as "baroclinic zones".<ref name="ExtraLessonMillUni">{{cite web |title=ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones |author=Dr. DeCaria |agency=Department of Earth Sciences, [[Millersville University]] |date=2005-12-07 |url=http://www.atmos.millersville.edu/~adecaria/ESCI241/esci241_lesson16_cyclones.html |access-date=2006-10-21 |archive-url=https://web.archive.org/web/20060903093420/http://www.atmos.millersville.edu/~adecaria/ESCI241/esci241_lesson16_cyclones.html |archive-date=2006-09-03 |url-status=dead 
}}</ref>

The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where [[extratropical transition]] has occurred,<ref name="ExtraLessonMillUni" /><ref name="ExtratropicalPhases">{{cite web |title=Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space |author1=Robert Hart |author2=Jenni Evans |agency=[[American Meteorological Society]] |year=2003 |url=http://ams.confex.com/ams/pdfpapers/70524.pdf |access-date=2006-10-03 }}</ref> but are often described as "depressions" or "lows" by weather forecasters and the public. These are the everyday phenomena that, along with [[anticyclone]]s, drive the weather over much of the Earth.

Although extratropical cyclones are almost always classified as [[baroclinic]] since they form along zones of temperature and dew point gradient within the [[westerlies]], they can sometimes become [[barotropic]] late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.<ref>Ryan N. Maue. [http://www.coaps.fsu.edu/~maue/cyclone_ch3.html CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS.] {{webarchive|url=https://web.archive.org/web/20080510210146/http://www.coaps.fsu.edu/~maue/cyclone_ch3.html |date=2008-05-10 }} Retrieved on 15 June 2008.</ref> An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core.<ref name="AOML FAQ A7">{{cite web |author=[[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division |title=Frequently Asked Questions: What is an extra-tropical cyclone? |publisher=[[NOAA]] |access-date=2006-07-25 |url=http://www.aoml.noaa.gov/hrd/tcfaq/A7.html}}</ref>

==Surface high-pressure systems==
{{Main|High-pressure area}}
[[Image:GGB in fog 2007 edit.jpg|thumb|right|250 px|[[Golden Gate Bridge]] in fog]]
High-pressure systems are frequently associated with light winds at the surface and [[Subsidence (atmosphere)|subsidence]] through the lower portion of the [[troposphere]].  Subsidence will generally dry out an air mass by [[adiabatic]], or compressional, heating.<ref>Office of the Federal Coordinator for Meteorology (2006). [http://www.ofcm.gov/fmh3/pdf/15-app-g.pdf Appendix G: Glossary.] {{webarchive|url=https://web.archive.org/web/20090225025158/http://www.ofcm.gov/fmh3/pdf/15-app-g.pdf |date=2009-02-25 }} [[NOAA]]. Retrieved on 2009-02-16.</ref>  Thus, high pressure typically brings clear skies.<ref>Jack Williams (2007). [https://www.usatoday.com/weather/tg/whighlow/whighlow.htm What's happening inside highs and lows.] [[USA Today]]. Retrieved on 2009-02-16.</ref>  During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave [[solar radiation]] and temperatures rise.  At night, the absence of clouds means that [[outgoing longwave radiation]] (i.e. heat energy from the surface) is not absorbed, giving cooler [[Diurnal temperature variation|diurnal]] low temperatures in all seasons.  When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread [[haze]].<ref>Myanmar government (2007). [http://www.kjc.gov.my/english/education/weather/haze01.html Haze.] {{webarchive|url=https://web.archive.org/web/20070127214023/http://www.kjc.gov.my/english/education/weather/haze01.html |date=2007-01-27 }} Retrieved on 2007-02-11.</ref>  If the low level [[relative humidity]] rises towards 100 percent overnight, [[fog]] can form.<ref>Robert Tardif (2002). [http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm Fog characteristics.] {{webarchive|url=https://web.archive.org/web/20110520021633/http://www.rap.ucar.edu/staff/tardif/Documents/CUprojects/ATOC5600/fog_characteristics.htm |date=2011-05-20 }} [[NCAR]] National Research Laboratory. Retrieved on 2007-02-11.</ref>

Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses.<ref>[[CBC News]] (2009). [https://www.cbc.ca/news/canada/north/blame-yukon-arctic-air-mass-chills-rest-of-north-america-1.860626?ref=rss Blame Yukon: Arctic air mass chills rest of North America.] Canadian Broadcasting Centre. Retrieved on 2009-02-16.</ref>  The low, sharp [[inversion (meteorology)|inversion]] can lead to areas of persistent [[stratocumulus]] or [[stratus cloud]], colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming.

On weather maps, these areas show converging winds (isotachs), also known as [[confluence]], or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere.<ref>Glossary of Meteorology (2009). [http://amsglossary.allenpress.com/glossary/search?id=level-of-nondivergence1 Level of nondivergence.] [[American Meteorological Society]]. Retrieved on 2009-02-17.</ref><ref>Konstantin Matchev (2009). [http://www.phys.ufl.edu/~matchev/MET1010/notes/Chapter12b.ppt Middle-Latitude Cyclones - II.] {{webarchive|url=https://web.archive.org/web/20090225025157/http://www.phys.ufl.edu/~matchev/MET1010/notes/Chapter12b.ppt |date=2009-02-25 }} [[University of Florida]]. Retrieved on 2009-02-16.</ref>  High-pressure systems are alternatively referred to as anticyclones.  On weather maps, high-pressure centers are associated with the letter H in English,<ref>Keith C. Heidorn (2005). [http://www.islandnet.com/~see/weather/elements/high.htm Weather's Highs and Lows: Part 1 The High.] The Weather Doctor. Retrieved on 2009-02-16.</ref> or A in Spanish,<ref>Instituto Nacional de Meteorologia. [http://www.inm.es/web/sup/ciencia/divulga/pdf/meteo_lapalma.pdf Meteorologia del Aeropuerto de la Palma.] {{webarchive|url=https://web.archive.org/web/20080309140653/http://www.inm.es/web/sup/ciencia/divulga/pdf/meteo_lapalma.pdf |date=2008-03-09 }} Retrieved on 2007-05-05.</ref> because alta is the Spanish word for high, within the [[isobar (meteorology)|isobar]] with the highest pressure value.  On constant pressure upper level charts, it is located within the highest height line contour.<ref name="AMS">Glossary of Meteorology (2009). [http://amsglossary.allenpress.com/glossary/search?id=high1 High.] [[American Meteorological Society]]. Retrieved on 2009-02-16.</ref>

==Weather fronts==
{{Main|Weather front}}
[[Image:Airmassesorigin.png|250px|thumb|right|Different air masses tend to be separated by frontal boundaries. The Arctic front separates Arctic from Polar air masses, while the Polar front separates Polar air from warm air masses. (cA is continental arctic; cP is continental polar; mP is maritime polar; cT is continental tropic; and mT is maritime tropic.)]]
A [[weather front]] is a boundary separating two [[air mass|masses of air]] of different [[densities]], and is the principal cause of [[meteorological phenomena]]. In [[surface weather analysis|surface weather analyses]], fronts are depicted using various colored lines and symbols, depending on the type of front.  The air masses separated by a front usually differ in [[temperature]] and [[humidity]].
Cold fronts may feature narrow bands of [[thunderstorm]]s and [[severe weather]], and may on occasion be preceded by [[squall line]]s or [[dry line]]s. [[Warm front]]s are usually preceded by [[Stratus cloud|stratiform]] [[precipitation]] and [[fog]]. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.<ref name=stm>{{cite web|title=Lesson 7: Clouds and Precipitation |access-date=2007-04-29 |url=http://www.ccrc.sr.unh.edu/~stm/AS/Teaching/STEC521/STEC521_7.html |publisher=Self-published |url-status=dead |archive-url=https://web.archive.org/web/20050111070558/http://www.ccrc.sr.unh.edu/~stm/AS/Teaching/STEC521/STEC521_7.html |archive-date=January 11, 2005 }}</ref>

Cold fronts and [[occluded front]]s generally move from west to east, while warm fronts move [[Geographical pole|poleward]]. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. [[Mountain]]s and warm bodies of water can slow the movement of fronts.<ref name="DR">{{cite web|author=David Roth|title=Unified Surface Analysis Manual|access-date=2006-10-22|publisher=[[Hydrometeorological Prediction Center]]|url= http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf}}</ref>  When a front becomes [[stationary front|stationary]], and the density contrast across the frontal boundary vanishes, the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline.  This is most common over the open ocean.

==See also==
* [[Mesoscale meteorology]]
* [[Microscale meteorology]]
* [[Misoscale meteorology]]
* [[Outline of meteorology]]

==References==
{{Reflist|2}}

==External links==
* [http://nsidc.org/cryosphere/glossary/term/synoptic-scale Definition of synoptic scale]
* [http://nsidc.org/cryosphere/glossary/term/synoptic-meteorology Definition of synoptic meteorology]
* [http://nsidc.org/cryosphere/glossary/term/synoptic-hour Definition of synoptic hours]

{{Cyclones}}
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[[Category:Synoptic meteorology and weather| ]]
[[Category:Atmospheric dynamics]]