Weather front 2188689 224291245 2008-07-08T05:02:24Z 70.254.6.242 /* Cold front */ - Removed unnecessary link for "cold front" at the beginning of the paragraph. [[Image:Airmassesorigin.gif|300px|thumb|right|Different air masses which affect North America, as well as other continents, tend to be separated by frontal boundaries. In this illustration, the Arctic front separates Arctic from Polar air masses, while the Polar front separates Polar air from warm air masses.]] A '''weather front''' is a boundary separating two [[air mass|masses of air]] of different [[density|densities]], and is the principal cause of [[meteorological phenomenon|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 [[stratiform]] [[Precipitation (meteorology)|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|author=Author unknown|title=Lesson 7: Clouds and Precipitation|year=|accessdate=2007-04-29|url=http://www.ccrc.sr.unh.edu/~stm/AS/Teaching/STEC521/STEC521_7.html|publisher=Self-published}}</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|year=|accessdate=2006-10-22|publisher=[[Hydrometeorological Prediction Center]]|url= http://www.hpc.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. ==Bergeron classification of air masses== [[Image:Air masses 2.jpg|thumb|250px|Source regions of global air masses]] {{Main article|Air mass}} The Bergeron classification is the most widely accepted form of air mass classification. Air mass classification involves three letters. The first letter describes its moisture properties, with c used for continental air masses (dry) and m for maritime air masses (moist). The second letter describes the thermal characteristic of its source region: T for tropical, P for polar, A for arctic or antarctic, M for monsoon, E for equatorial, and S for superior air (dry air formed by significant downward motion in the atmosphere). The third letter is used to designate the stability of the atmosphere. If the air mass is colder than the ground below it, it is labeled k. If the air mass is warmer than the ground below it, it is labeled w.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=airmass-classification1 Airmass Classification.] Retrieved on [[2008-05-22]].</ref> ==Surface weather analysis== [[Image:NWS weather fronts.svg|thumb|right|180px|Weather map symbols: 1.&nbsp;cold front; 2.&nbsp;warm front; 3.&nbsp;stationary front; 4.&nbsp;occluded front; 5.&nbsp;surface trough; 6.&nbsp;squall/shear line; 7.&nbsp;dry line; 8.&nbsp;tropical wave]] {{Main|Surface weather analysis|Low pressure area|High pressure area}} A surface weather analysis is a special type of [[weather map]] which provides a view of weather elements over a geographical area at a specified time based on information from ground&ndash;based weather stations.<ref name="disc1">Monmonier, Mark. ''Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather.'' University of Chicago Press. Chicago: 1999.</ref> Weather maps are created by plotting or tracing the values of relevant quantities such as [[Atmospheric pressure|sea-level pressure]], [[temperature]], and [[cloud cover]] onto a [[Cartography|geographical map]] to help find [[Synoptic scale meteorology|synoptic scale]] features such as weather fronts. Surface weather analyses have special symbols which show frontal systems, cloud cover, [[Precipitation (meteorology)|precipitation]], or other important information. For example, an ''H'' may represent high pressure, implying fair weather. An ''L'' on the other hand may represent 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. In addition, areas of precipitation help determine the frontal type and location.<ref name="disc1">''Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather.'' University of Chicago Press. Chicago: 1999.</ref> ==Front types== ===Cold front=== {{Main|Cold front}} [[Image:Example of a cold front.svg|180px|right|thumb|Illustration of a cold front]] A cold front is located at the leading edge of the temperature drop off, which in an [[Contour line|isotherm]] analysis shows up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface [[trough (meteorology)|trough]]. Cold fronts can move up to twice as fast and produce sharper changes in weather than warm fronts, since cold air is denser than warm air and rapidly replaces the warm air preceding the boundary. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangle-shaped pips pointing in the direction of travel, and it is placed at the leading edge of the cooler air mass.<ref name="DR"/> Cold fronts come in association with a [[low pressure area]]. When a cold front moves through, the air with greater density wedges under the less dense warmer air, lifting it, which can cause the formation of a narrow line of [[showers]] and [[thunderstorms]] when enough [[moisture]] is present. This upward motion causes lowered [[pressure]] along the cold front. <ref name="DR"/> ===Warm front=== {{Main|Warm front}} [[Image:Example of a warm front.svg|180px|thumb|left|Illustration of a warm front]] Warm fronts are at the leading edge of a homogeneous warm air mass, which is located on the equatorward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. A warm front moves more slowly than the cold front which usually follows because cold air is more dense and harder to remove from the earth's surface.<ref name="DR"/> This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly [[Stratus cloud|stratiform]], and rainfall gradually increases as the front approaches. [[Fog]] can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semi-circles pointing in the direction of travel.<ref name="DR"/> ===Occluded front=== {{Main|Occluded front}} [[Image:Occluded_cyclone.svg|180px|thumb|right|Occluded cyclone front depiction]] An [[occluded front]] is formed when a cold front overtakes a warm front.<ref>{{cite web|publisher=University of Illinois Department of Atmospheric Sciences|title=Occluded Front|year=|accessdate=2006-10-22|url=http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/ofdef.rxml}}</ref> The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point.<ref>{{cite web|author=National Weather Service Office, Norman, Oklahoma|title=Triple Point|year=|publisher=NOAA|accessdate=2006-10-22|url=http://www.srh.noaa.gov/oun/severewx/glossary4.php#t}}</ref> It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold. 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 warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air.<ref name="DR"/> A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. 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 mature low-pressure areas.<ref name="DR"/> ===Stationary front and shearline=== {{Main|Stationary front}} A [[stationary front]] is a non-moving (or stalled) boundary between two air masses, neither of which is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, usually moving in waves.<ref> [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/sfdef.rxml Stationary Front.] University of Illinois Department of Atmospheric Sciences. Retrieved on [[2006-10-22]].</ref> There is normally a broad [[temperature gradient]] behind the boundary with more widely spaced [[isotherm]] packing. A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts either dissipate after several days or devolve into shear lines, but they can transform into a cold or warm front if conditions aloft change. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement. When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shearlines.<ref>{{cite web|work=Glossary of Meteorology|title=Shear Line|year=|accessdate=2006-10-22|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line|publisher=American Meteorological Society|publisher=American Meteorological Society}}</ref> A shearline is depicted as a line of red dots and dashes.<ref name="DR"/> ===Dry line=== {{Main|Dry line}} A similar phenomenon to a weather front is the [[dry line]], which is the boundary between air masses with significant moisture differences. When westerly winds aloft increase on the north side of surface highs, areas of lowered [[Barometric pressure|pressure]] will form downwind of north–south oriented mountain chains, leading to the formation of a lee trough. Near the surface during daylight hours, warm moist air is more dense than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the dry air and the boundary slope reverses. In the vicinity of the reversal aloft, [[severe weather]] is possible, especially when a triple point is formed with a cold front.<ref>{{cite web|author=Huaqing Cai|title=Dryline cross section|year=|accessdate=2006-12-05|url=http://www.atmos.ucla.edu/~caihq/pic/fig23.html}}</ref> A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in [[moisture]]. When moisture pools along the boundary during the warm season, it can be the focus of diurnal [[thunderstorms]].<ref>{{cite web|work=Glossary of Meteorology|title=Lee Trough|year=|accessdate=2006-10-22|url=http://amsglossary.allenpress.com/glossary/search?id=lee-trough1|publisher=American Meteorological Society}}</ref> The dry line may occur anywhere on earth in regions intermediate between [[desert]] areas and warm seas. The southern plains west of the [[Mississippi River]] in the [[United States]] are a particularly favored location. The dry line normally moves eastward during the day and westward at night. A dry line is depicted on [[National Weather Service]] (NWS) surface analyses as a orange line with scallops facing into the moist sector. Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion.<ref>{{cite web|publisher=University of Illinois Department of Atmospheric Science|title=Dry Line: A Moisture Boundary|year=|accessdate=2006-10-22|url=http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/dfdef.rxml}}</ref> ===Squall line=== {{Main article|Squall line}} [[Image:Shelfcloudspc.jpg|thumb|right|250px|A shelf cloud such as this one can be a sign that a squall is imminent]] Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but can outrun cold fronts in a pattern where the upper level jet splits apart into two streams, with the resultant [[Mesoscale convective system|Mesoscale Convective System]] (MCS) forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise.<ref>{{cite web|author=Office of the Federal Coordinator for Meteorology|title=Chapter 2: Definitions|year=|accessdate=2006-10-22|url=http://www.ofcm.gov/slso/pdf/slsochp2.pdf}}</ref> Even weaker and less organized areas of thunderstorms lead to locally cooler air and higher pressures, and outflow boundaries exist ahead of this type of activity, which can act as foci for additional thunderstorm activity later in the day.<ref>{{cite web|author=Michael Branick|title=A Comprehensive Glossary of Weather|year=|publisher=National Weather Service Office, Norman, Oklahoma|accessdate=2006-10-22|url=http://www.geographic.org/climate/o.html|publisher=American Meteorological Society}}</ref> These features are often depicted in the warm season across the United States on surface analyses and lie within surface troughs. If outflow boundaries or squall lines form over arid regions, a [[haboob]] may result.<ref>{{cite web|author=Western Region Climate Center|title=H|year=|accessdate=2006-10-22|url=http://www.wrcc.dri.edu/ams/glossary.html#H}}</ref> Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or SQUALL LINE, while outflow boundaries are depicted as troughs with a label of OUTFLOW BNDRY. ===Tropical waves=== {{Main|Tropical wave}} [[Image:Tropical waves.jpg|thumb|right|Tropical wave formation]] Atlantic [[tropical wave]]s develop from disturbances which drift off the continent of [[Africa]] onto the [[Atlantic Ocean]]. They are generated or enhanced by the [[African Easterly Jet]]. The clockwise circulation of the large transoceanic [[High pressure area|high-pressure cell]] or [[anticyclone]] centered near the [[Azores]] islands moves easterly waves away from the coastal areas of Africa towards [[North America]]. Tropical waves cause approximately&nbsp;60% of Atlantic [[tropical cyclone]]s and&nbsp;85% of intense Atlantic hurricanes ([[Saffir-Simpson Hurricane Scale|Category&nbsp;3]] and greater).<ref name = "AOML FAQ A4">{{ cite web | author = [[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division | title = Frequently Asked Questions: What is an easterly wave? | publisher = [[NOAA]] | accessdate = 2006-07-25 | url = http://www.aoml.noaa.gov/hrd/tcfaq/A4.html}}</ref><ref name="MWR Avila 1995">{{cite journal | last = [[Lixion Avila|Avila, Lixion]] | coauthors = Richard Pasch | year = 1995 | month = March | title = Atlantic tropical systems of 1993 | journal = [[Monthly Weather Review]] | volume = 123 | issue = 3 | pages = 887–896 | url = http://ams.allenpress.com/perlserv/?request=res-loc&uri=urn%3Aap%3Apdf%3Adoi%3A10.1175%2F1520-0493%281995%29123%3C0887%3AATSO%3E2.0.CO%3B2 | format = PDF | accessdate = 2006-07-25 | doi = 10.1175/1520-0493(1995)123<0887:ATSO>2.0.CO;2 }}</ref> Tropical cyclones sometimes degenerate back into a tropical wave. This normally occurs if upper-level [[wind shear]] is too strong. The storm can redevelop if the upper level shear abates. If a tropical wave is moving quickly, it can have strong winds of over tropical storm force but is not considered a tropical storm unless it has a closed circulation. An example of this was [[Hurricane Claudette (2003)|Hurricane Claudette]] in&nbsp;2003, where the original wave had winds of {{convert|45|mi/h|km/h|abbr=on}} before developing a circulation. Tropical waves are depicted with a solid orange line on the U.S. National Weather Service Unified Surface Analysis.<ref name="DR"/> == Precipitation produced == {{main|Precipitation (meteorology)}} [[Image:Konvektionsregen.jpg|right|200px|thumb|Convective precipitation]] Fronts are the principal cause of significant weather. ''Convective precipitation'' (showers, thundershowers, and related unstable weather) is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme because of [[wind shear]] and the presence of a strong [[jet stream]], "[[roll cloud]]s" and [[tornado]]es may occur.<ref>{{cite web|work=Glossary of Meteorology|title=Convection|year=|accessdate=2006-10-22|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=convection|publisher=American Meteorological Society}}</ref> In the warm [[season]], lee troughs, breezes, outflow boundaries and occlusions can lead to convection if enough moisture is available. ''Orographic precipitation'' is precipitation created through the lifting action of air moving over terrain such as mountains and hills, which is most common behind cold fronts that move into mountainous areas. It may sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain. However, precipitation along warm fronts is relatively steady, as in rain or drizzle. Fog, sometimes extensive and dense, often occurs in pre-warm-frontal areas.<ref>{{cite web|work=Glossary of Meteorology|title=Orographic Lifting|year=|accessdate=2006-10-22|url=http://amsglossary.allenpress.com/glossary/search?id=orographic-lifting1|publisher=American Meteorological Society}}</ref> Although, not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted.<ref name=stm/> == Movement == Fronts are generally guided by winds aloft, but do not move as quickly. Cold fronts and occluded fronts in the Northern Hemisphere usually travel from the northwest to southeast, while warm fronts move more poleward with time. In the Northern Hemisphere a warm front moves from southwest to northeast. In the Southern Hemisphere, the reverse is true; a cold front usually moves from southwest to northeast, and a warm front moves from northwest to southeast. Movement is largely caused by the pressure gradient force (horizontal differences in atmospheric pressure) and the [[Coriolis effect]], which is caused by [[Earth]]'s spinning about its axis. Frontal zones can be slowed down by geographic features like mountains and large bodies of warm water.<ref name="DR">David Roth. Hydrometeorological Prediction Center. [http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf Unified Surface Analysis Manual.] Retrieved on 2006-10-22.</ref> == See also == * [[Cyclogenesis]] * [[Extratropical cyclone]] * [[Norwegian cyclone model]] * [[Surface weather analysis]] ==References== {{reflist}} ==Bibliography== * (1999). ''Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather''. University of Chicago Press, Chicago. ==External links== *[http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf Surface Analysis Manual] *[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/cfrnt/def.rxml Cold Fronts] *[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/wfrnt/def.rxml Warm Fronts] *[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/home.rxml Fronts: the boundaries between air masses] [[Category:Basic meteorological concepts and phenomena]] [[Category:Synoptic meteorology and weather]] [[Category:Weather fronts|*]] [[ar:جبهة الطقس]] [[ca:Front (meteorologia)]] [[cs:Atmosférická fronta]] [[de:Front (Meteorologie)]] [[es:Frente (meteorología)]] [[fa:جبهه هوا]] [[fr:Front (météorologie)]] [[gl:Fronte (meteoroloxía)]] [[he:חזית מזג אוויר]] [[lt:Atmosferos frontas]] [[hu:Légköri front]] [[nl:Front (meteorologie)]] [[ja:前線 (気象)]] [[no:Værfront]] [[nn:Vêrfront]] [[pl:Front meteorologiczny]] [[pt:Sistema frontal]] [[ru:Атмосферный фронт]] [[sk:Front (počasie)]] [[fi:Säärintama]] [[uk:Фронт атмосферний]] [[zh:锋]]