Editing Contour line (section)

==== Barometric pressure ====<!-- [[Isobar (meteorology)]] redirects here -->
[[File:Loop isallobaric tendencies.gif|thumb|Video loop of isallobars showing the motion of a [[cold front]]]]
An '''isobar''' ({{ety|grc|''βάρος'' (baros)|weight}}) is a line of equal or constant [[pressure]] on a graph, plot, or map; an isopleth or contour line of pressure. More accurately, isobars are lines drawn on a map joining places of equal average atmospheric pressure reduced to sea level for a specified period of time. In [[meteorology]], the [[barometric pressure]]s shown are reduced to [[sea level]], not the surface pressures at the map locations.<ref>{{cite web |publisher=University of Wisconsin |author=Edward J. Hopkins, Ph.D. |date=1996-06-10 |url=http://www.meteor.wisc.edu/~hopkins/aos100/sfc-anl.htm |title=Surface Weather Analysis Chart |access-date=2007-05-10}}</ref>  The distribution of isobars is closely related to the magnitude and direction of the [[wind]] field, and can be used to predict future weather patterns. Isobars are commonly used in television weather reporting.

'''Isallobars''' are lines joining points of equal pressure change during a specific time interval.<ref name="OMM">{{cite web | url= http://www.eumetcal.org/resources/ukmeteocal/rapid_cyclo/www/english/glossary/isalloba.htm | title= Isallobar | author= World Meteorological Organisation | author-link= World Meteorological Organisation | work= Eumetcal | access-date= 12 April 2014 | url-status= dead | archive-url= https://web.archive.org/web/20140416031654/http://www.eumetcal.org/resources/ukmeteocal/rapid_cyclo/www/english/glossary/isalloba.htm | archive-date= 16 April 2014 }}</ref> These can be divided into ''anallobars'', lines joining points of equal pressure increase during a specific time interval,<ref name="OMM-1">{{cite web | url= http://www.eumetcal.org/euromet/glossary/analloba.htm | archive-url= https://web.archive.org/web/20150924003018/http://www.eumetcal.org/euromet/glossary/analloba.htm | url-status= dead | archive-date= 24 September 2015 | title= Anallobar | author= World Meteorological Organisation | author-link= World Meteorological Organisation | work= Eumetcal | access-date= 12 April 2014 }}</ref> and ''katallobars'', lines joining points of equal pressure decrease.<ref name="OMM-2">{{cite web | url= http://www.eumetcal.org/euromet/glossary/katallob.htm | archive-url= https://web.archive.org/web/20080205124154/http://www.eumetcal.org/euromet/glossary/katallob.htm | url-status= dead | archive-date= 5 February 2008 | title= Katallobar | author= World Meteorological Organisation | author-link= World Meteorological Organisation | work= Eumetcal | access-date= 12 April 2014 }}</ref> In general, weather systems move along an axis joining high and low isallobaric centers.<ref>{{ cite web | url= http://apollo.lsc.vsc.edu/classes/met130/notes/chapter13/isallobars.html | title= Forecasting weather system movement with pressure tendency | work= Chapter 13 – Weather Forecasting | publisher = Lyndon State College Atmospheric Sciences | access-date = 12 April 2014}}</ref> Isallobaric gradients are important components of the wind as they increase or decrease the [[geostrophic wind]].

An [[isopycnal]] is a line of constant density. An ''isoheight'' or ''isohypse'' is a line of constant [[geopotential]] height on a constant pressure surface chart. Isohypse and isoheight are simply known as lines showing equal pressure on a map.