Editing Surface weather analysis (section)

== Synoptic scale features ==
{{see also|Synoptic scale meteorology}}
A synoptic scale feature is one whose dimensions are large in scale, more than several hundred kilometers in length.<ref>Glossary of meteorology. [http://amsglossary.allenpress.com/glossary/search?id=synoptic-scale1 Synoptic scale.] {{webarchive|url=https://web.archive.org/web/20070811102318/http://amsglossary.allenpress.com/glossary/search?id=synoptic-scale1 |date=2007-08-11 }} Retrieved on 2007-05-10.</ref> Migratory pressure systems and frontal zones exist on this scale.{{Citation needed|date=December 2021}}

=== Pressure centers ===
[[Image:Wind barbs.gif|thumb|upright|Wind barb interpretation]]

Centers of surface high- and low-pressure areas that are found within closed isobars on a surface weather analysis are the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps in English-speaking countries will depict their highs as Hs and lows as Ls,<ref>Weather Doctor. [http://www.islandnet.com/~see/weather/elements/high.htm Weather's Highs and Lows: Part 1 The High.]</ref> while Spanish-speaking countries will depict their highs as As and lows as Bs.<ref>Agencia Estatal de Meteorología. [http://www.aemet.es/en/divulgacion/aeronautica/detalles/Meteorologia_del_aeropuerto_de_La_Palma Meteorología del aeropuerto de La Palma.].</ref>

==== Low pressure ====
Low-pressure systems, also known as [[cyclone]]s, are located in minima in the pressure field.  Rotation is inward at the surface and counterclockwise in the [[northern hemisphere]] as opposed to inward and clockwise in the [[southern hemisphere]] due to the [[Coriolis force]]. Weather is normally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds, increased temperatures, and upward motion in the atmosphere, which leads to an increased chance of precipitation. [[Polar low]]s can form over relatively mild ocean waters when cold air sweeps in from the ice cap. The relatively warmer water leads to upward convection, causing a low to form, and precipitation usually in the form of snow. Tropical cyclones and winter storms are intense varieties of low pressure. Over land, [[thermal low]]s are indicative of hot weather during the summer.<ref>BBC Weather. [http://www.bbc.co.uk/weather/features/basics_lowpressure.shtml Weather Basics - Low Pressure.] Retrieved on 2007-05-05.</ref>

==== High pressure ====
High-pressure systems, also known as [[anticyclone]]s, rotate outward at the surface and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking of the atmosphere slightly warms the air by compression, leading to clearer skies, winds that are lighter, and a reduced chance of precipitation.<ref>BBC Weather. [http://www.bbc.co.uk/weather/features/understanding/highpressure.shtml High Pressure.] Retrieved on 2007-05-05.</ref> The descending air is dry, hence less energy is required to raise its temperature. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high.<ref>United Kingdom School System. [http://atschool.eduweb.co.uk/radgeog/metlink/ppt/highs/Press-System.ppt Pressure, Wind and Weather Systems.] {{webarchive|url=https://web.archive.org/web/20070927190000/http://atschool.eduweb.co.uk/radgeog/metlink/ppt/highs/Press-System.ppt |date=2007-09-27 }} Retrieved on 2007-05-05.</ref>

=== Fronts ===<!-- This section is linked from [[Jet stream]] -->
{{Main|Weather front}}
[[Image:Occluded cyclone.svg|thumb|Occluded cyclone example. The triple point is the intersection of the cold, warm, and occluded [[weather front|fronts]].]]

Fronts in meteorology are boundaries between [[air mass]]es that have different density, air temperature, and [[humidity]]. Strictly speaking, the front is marked at the warmer edge of a ''frontal zone'' where the [[gradient]] is very large. When a front passes over a point, it is marked by changes in temperature, moisture, wind speed and direction, a minimum of atmospheric pressure, and a change in the cloud pattern, sometimes with precipitation. [[Cold front]]s develop where the cold air mass is advancing, [[warm front]]s where the warm air is advancing, and a [[stationary front]] is not moving. Fronts classically wrap around low pressure centers as indicated in the [[:File:Occluded cyclone.svg|image]] here depicted for the northern hemisphere. On a larger scale, the Earth's [[polar front]] is a sharpening of the general equator-to-pole temperature gradient, underlying a high-altitude [[jet stream]] for reasons of [[Thermal wind|thermal wind balance]]. Fronts usually travel from west to east, although they can move in a north-south direction or even east to west (a [[backdoor cold front|"backdoor" front]]) as airflow wraps around a low pressure center. Frontal zones can be distorted by such geographic features as mountains and large bodies of water.<ref name="DR"/>

==== Cold front ====
{{Main|Cold front}}
A cold front is located at the leading edge of a sharp temperature gradient on an [[Isotherm (contour line)|isotherm]] analysis, often marked by a sharp surface pressure [[Trough (meteorology)|trough]]. Cold fronts can move up to twice as quickly as warm fronts and produce sharper changes in [[weather]] since cold air is denser than warm air and rapidly lifts as well as pushes the warmer air. Cold fronts are typically accompanied by a narrow band of clouds, showers and thunderstorms. On a weather map, the surface position of the cold front is marked with a blue line of triangles (pips) pointing in the direction of travel, at the leading edge of the cooler air mass.<ref name="DR"/>

==== Warm front ====

{{Main|Warm front}}

[[Warm front]]s mark the position on the Earth's surface where a relatively warm body of air is advancing into colder air. The front is marked on the warm edge of the gradient in isotherms, and lies within a low pressure trough that tends to be broader and weaker than that of a cold front. Warm fronts move more slowly than cold fronts because cold air is denser, and is only pushed along (not lifted from) the Earth's surface. The warm air mass overrides the cold air mass, so temperature and cloud changes occur at higher altitudes before those at the surface. Clouds ahead of the warm front are mostly [[Stratus cloud|stratiform]] with precipitation that increases gradually as the front approaches. Ahead of a warm front, descending cloud bases will often begin with [[Cirrus cloud|cirrus]] and [[Cirrostratus cloud|cirrostratus]] (high-level), then [[Altostratus cloud|altostratus]] (mid-level) clouds, and eventually lower in the atmosphere as the front passes through. [[Fog]] can precede a warm front when precipitation falls into areas of colder air, but increasing surface temperatures and wind tend to dissipate it after a warm front passes through. Cases with environmental [[Convective instability|instability]] can be conducive to thunderstorm development. On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of travel.

[[Image:Warmfrontai.svg|thumb|Illustration clouds overriding a [[warm front]]]]

==== Occluded front ====
{{Main|Occluded front}}The classical view of an [[occluded front]] is that they are formed when a cold front overtakes a warm front.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/ofdef.rxml Occluded Front.] Retrieved on 2006-10-22.</ref> A more modern view suggests that they form directly during the wrap-up of the [[Baroclinity|baroclinic zone]] during [[cyclogenesis]], and lengthen due to flow [[Deformation (meteorology)|deformation]] and rotation around the cyclone.<ref>{{Cite journal|last1=Schultz|first1=David M.|last2=Vaughan|first2=Geraint|date=2011-04-01|title=Occluded Fronts and the Occlusion Process: A Fresh Look at Conventional Wisdom|journal=Bulletin of the American Meteorological Society|language=en|volume=92|issue=4|pages=443–466|doi=10.1175/2010BAMS3057.1|bibcode=2011BAMS...92..443S|issn=0003-0007|doi-access=free}}</ref>

Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel: that is, with a mixture of warm and cold frontal colors and symbols. Occlusions can be divided into warm vs. cold types.<ref>{{Cite journal|last1=Stoelinga|first1=Mark T.|last2=Locatelli|first2=John D.|last3=Hobbs|first3=Peter V.|title=Warm Occlusions, Cold Occlusions, and Forward-Tilting Cold Fronts |date=2002-05-01|journal=Bulletin of the American Meteorological Society|language=en|volume=83|issue=5|pages=709–722|doi=10.1175/1520-0477(2002)083<0709:WOCOAF>2.3.CO;2|bibcode=2002BAMS...83..709S |issn=0003-0007|doi-access=free}}</ref> In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel.<ref name="DR"/>

Occluded fronts usually form around low pressure systems in the mature or late stages of their life cycle, but some continue to deepen after occlusion, and some do not form occluded fronts at all. The weather associated with an occluded front includes a variety of cloud and precipitation patterns, including dry slots and banded precipitation. Cold, warm and occluded fronts often meet at the point of occlusion or triple point.<ref>National Weather Service Office, Norman, Oklahoma. [http://www.srh.noaa.gov/oun/severewx/glossary4.php#t Triple Point.] Retrieved on 2006-10-22. {{webarchive|url=https://web.archive.org/web/20061009165650/http://www.srh.noaa.gov/oun/severewx/glossary4.php#t|date=October 9, 2006}}</ref> [[Image:NWS weather fronts.svg|thumb|A guide to the symbols for [[weather fronts]] that may be found on a weather map:<br />
1. cold front<br />
2. warm front<br />
3. stationary front<br />
4. occluded front<br />
5. surface trough<br />
6. squall line<br />
7. dry line<br />
8. tropical wave<br />
9. Trowal]]

==== Stationary fronts and shearlines ====
{{Main|Stationary front}}
A stationary front is a non-moving boundary between two different air masses. They tend to remain in the same area for long periods of time, sometimes undulating in waves.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/sfdef.rxml Stationary Front.] Retrieved on 2006-10-22.</ref> Often a less-steep temperature gradient continues behind (on the cool side of) the sharp frontal zone with more widely spaced isotherms. A wide variety of weather can be found along a stationary front, characterized more by its prolonged presence than by a specific type. Stationary fronts may dissipate after several days, but can change into a cold or warm front if conditions aloft change, driving one air mass toward the other. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement.{{Citation needed|date=December 2021}}

As airmass temperatures equalize, stationary fronts may become smaller in scale, degenerating to a narrow zone where wind direction changes over a short distance, known as a shear line,<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line Shear Line.] {{webarchive|url=https://web.archive.org/web/20070314081220/http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line |date=2007-03-14 }} Retrieved on 2006-10-22.</ref> depicted as a blue line of single alternating dots and dashes.<ref name="DR"/><ref>Aviation Weather. [https://www.aviationweather.ws/093_Transitory_Systems.php] Retrieved on 2021-03-13.</ref>