View source for Warm front

{{short description|Boundary of advancing mass of warm air}}
[[File:Warm front.svg|thumb|center|600px|Illustration of a warm front. The warm air behind the front is slowly overtaking the cold air ahead of the front, which is moving more slowly in the same direction. The warmer air, due to lower density, rises over the colder air as it moves. As a result of its increased altitude, it cools off and its moisture condenses, forming clouds and possibly precipitation.]]
[[File:Warm front symbol.svg|thumb|Symbol commonly used to represent a warm front.]]
A '''warm front''' is a [[weather front|density discontinuity]] located at the leading edge of a homogeneous warm [[air mass]], and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than [[cold front]]s, and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface.<ref name="DR">{{cite web|author=David Roth|date=2006-12-14|title=Unified Surface Analysis Manual|access-date=2010-12-17|publisher=[[Hydrometeorological Prediction Center]]|url= http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf}}</ref>  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 generally 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 [[surface weather analysis|weather maps]], the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.<ref name="DR"/>
==Development==
[[Image:Airmassesorigin.png|250px|thumb|right|Different air masses that affect North America, as well as other continents, tend to be separated by frontal boundaries.]]
Air masses are large bodies of air with similar properties of [[temperature]] and [[humidity]] that form over source regions.  The warm air mass behind a warm front is not only warmer, but often (but not always) also higher in humidity than the colder air preceding it. Because of a warm air mass’s higher temperature and thus lesser [[density]], mixing between the two air masses is unlikely. Being light, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 1:200 and lifting is slow but persistent.
As the air mass rises into regions of lower pressure, it expands and cools. As it cools, any [[water vapor]] that is present will [[Condensation|condense]] and form extensive [[cloud]] cover.  The first clouds that indicate an approaching warm front tend to be mostly high [[Cirrus cloud|cirrus]] at first, changing to [[Cirrostratus cloud|cirrostratus]] as the front approaches. However, if [[Cirrocumulus cloud|cirrocumulus]] also appears, there is greater airmass instability approaching ahead of the front. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 8 hours away.<ref name="Justice Perrin">{{cite web|url=http://www.weatheronline.co.uk/reports/wxfacts/Sometimes-a-bit-fishy.htm|title=Mackerel sky|publisher=Weather Online|access-date=21 November 2013}}</ref>  A thickening and lowering of these high clouds into middle-stage [[Altostratus cloud|altostratus]] or [[Altocumulus cloud|altocumulus]] is a good sign the warm front or low has moved closer and precipitation may begin within less than six hours.  Once the clouds have thickened to {{convert|2500|m|ft}} from the earth’s surface, precipitation can begin to fall from heavy [[nimbostratus cloud|nimbostratus]].  If unstable altocumulus  castellanus accompanies or takes the place of the main altostratus layer, cumulus congestus or cumulonimbus producing showers or thunderstorms may follow.  Low stratus and stratocumulus commonly form underneath the main precipitating clouds.
A warm front is also defined as the transition zone where a warmer air mass is replacing a cooler air mass. Warm fronts generally move from southwest to northeast.  If the warmer air originates over the ocean, it is not only warmer but also more moist than the air ahead of it.
==Characteristics==
If the air mass is relatively stable, [[rainfall]] will increase until the front reaches the location, at which time the clouds can extend all the way to the earth’s surface as [[fog]].  Once the front passes, the location experiences some warming and clearing.  If the [[air mass]] is unstable, [[thunderstorm]]s may precede and follow the front and temperature changes will be larger.<ref>{{cite book|url=https://books.google.com/books?id=Ew3MBjbw4OAC&q=anafront+book&pg=PA309|title=The environment: principles and applications|author=Chris C. Park|page=309|publisher=Psychology Press|year=2001|access-date=2010-12-17|isbn=978-0-415-21771-2}}</ref>
In the [[northern hemisphere]], a warm front causes a shift of [[wind]] blowing from southeast to southwest, and in the [[southern hemisphere]] a shift from winds blowing from northeast to northwest. Common characteristics associated with warm fronts include:
{| class="wikitable"
|-
! Weather phenomenon
! Prior to the passing of the front
! While the front is passing
! After the passing of the front
|-
| Temperature
| Cool
| Warming suddenly
| Warmer, then leveling off
|-
| Atmospheric pressure
| Decreasing steadily
| Leveling off
| Slight rise followed by a decrease
|-
| Winds
|
* South to southeast (backing) (northern hemisphere)
* North to northeast (veering) (southern hemisphere)
| Variable
|
* South to southwest (veering) (northern hemisphere)
* North to northwest (backing) (southern hemisphere)
|-
| Precipitation
| Usually none, but in summer or warm temperatures, cumulus congestus may continue to exist under cirrostratus and altostratus creating light to moderate showers.
| Persistent rain, usually moderate with some lighter periods and some heavier bursts. In winter, snow may turn to rain after passing through [[ice pellets]] and [[freezing rain]].<ref>{{Cite book|title=Meteorology today : an introduction to weather, climate, and the environment|last=Donald|first=Ahrens, C.|date=2007|publisher=Thomson/Brooks/Cole|isbn=978-0495011620|edition= 8th|location=Belmont, Calif.|pages=298–300|oclc=66911677}}</ref>
| Light drizzle, gradually ceasing.
|-
| Clouds
| [[Cirrus cloud|Cirrus]], [[cirrostratus]], [[altostratus]], [[nimbostratus]],  then [[stratus cloud|stratus]].  Other clouds can also often be seen, including [[cirrocumulus]] amongst the approaching cirrus, [[altocumulus]] with or instead of altostratus (particularly if the front is weak), and occasionally [[cumulonimbus]] along with or instead of nimbostratus in summer.  Additionally, [[stratocumulus]] often appears underneath the main altostratus deck and stratus [[fractus]] typically forms in precipitation falling from the thick nimbostratus layer. Often in warm temperatures, rain bearing [[cumulus congestus]] clouds can appear under the cirrostratus, and more rarely [[altocumulus castellanus]] clouds if convection is sufficient.  In cold humid conditions, low airmass stratus or [[fog]] may obscure the main frontal clouds.
| [[Nimbostratus]], sometimes [[cumulonimbus]]
| Clearing with scattered [[stratus cloud|stratus]] and [[stratocumulus]]. If the warm front is part of a depression, there is often a sheet of altostratus (often broken in places to altocumulus) above this which thickens when the cold front approaches.
|-
| Visibility
| Poor
| Poor, but improving
| Sunny
|-
| [[Dew point]]
| Steady rise
| Steady
| Rise, then steady
|}
==Warm sector==
The warm sector is a near-surface [[air mass]] in between the warm front and the [[cold front]], on the equatorward side of an [[extratropical cyclone]]. With its warm and [[Humidity|humid]] characteristics, this air is susceptive to [[convective instability]] and can sustain [[Thunderstorm|thunderstorms]], especially if lifted by the advancing cold front.
[[Image:Surface analysis.gif|thumb|right|250px|A surface weather analysis for the [[United States]] on October 21, 2006.  Note the warm front in the northwest Gulf of Mexico.]]
==Depiction==
On [[weather map]]s, the surface location of a warm front is marked with a red line of half circles pointing in the direction of the front. On colored weather maps, warm fronts are illustrated with a solid red line.
==See also==
* [[Cold front]]
* [[Occluded front]]
* [[Pseudo-warm front]]
* [[Weather front]]
==References==
{{Reflist}}
==External links==
*[http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/wfrnt/def.rxml Warm Front: transition zone from cold air to warm air]
*[http://www.ace.mmu.ac.uk/eae/Weather/Older/Warm_Fronts.html Warm Fronts]
* {{Internet Archive film clip|id=gov.dod.dimoc.27503|description=AIR MASSES AND FRONTS - THE WARM FRONT (1962)}}
[[Category:Weather fronts]]