Warm front

Symbol commonly used to represent a warm front.

Awarm frontis adensity discontinuitylocated at the leading edge of a homogeneous warmair mass,and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure thancold fronts,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.^[1]This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostlystratiform,and rainfall generally increases as the front approaches.Fogcan 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. Onweather maps,the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.^[1]

Development

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 oftemperatureandhumiditythat 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 lesserdensity,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, anywater vaporthat is present willcondenseand form extensivecloudcover. The first clouds that indicate an approaching warm front tend to be mostly highcirrusat first, changing tocirrostratusas the front approaches. However, ifcirrocumulusalso 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.^[2]A thickening and lowering of these high clouds into middle-stagealtostratusoraltocumulusis 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 2,500 metres (8,200 ft) from the earth’s surface, precipitation can begin to fall from heavynimbostratus.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,rainfallwill increase until the front reaches the location, at which time the clouds can extend all the way to the earth’s surface asfog.Once the front passes, the location experiences some warming and clearing. If theair massis unstable,thunderstormsmay precede and follow the front and temperature changes will be larger.^[3]

In thenorthern hemisphere,a warm front causes a shift ofwindblowing from southeast to southwest, and in thesouthern hemispherea shift from winds blowing from northeast to northwest. Common characteristics associated with warm fronts include:

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 throughice pelletsandfreezing rain.^[4]	Light drizzle, gradually ceasing.
Clouds	Cirrus,cirrostratus,altostratus,nimbostratus,thenstratus.Other clouds can also often be seen, includingcirrocumulusamongst the approaching cirrus,altocumuluswith or instead of altostratus (particularly if the front is weak), and occasionallycumulonimbusalong with or instead of nimbostratus in summer. Additionally,stratocumulusoften appears underneath the main altostratus deck and stratusfractustypically forms in precipitation falling from the thick nimbostratus layer. Often in warm temperatures, rain bearingcumulus congestusclouds can appear under the cirrostratus, and more rarelyaltocumulus castellanusclouds if convection is sufficient. In cold humid conditions, low airmass stratus orfogmay obscure the main frontal clouds.	Nimbostratus,sometimescumulonimbus	Clearing with scatteredstratusandstratocumulus.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-surfaceair massin between the warm front and thecold front,on the equatorward side of anextratropical cyclone.With its warm andhumidcharacteristics, this air is susceptive toconvective instabilityand can sustainthunderstorms,especially if lifted by the advancing cold front.

Depiction

Onweather maps,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.

References

^^a ^bDavid Roth (2006-12-14)."Unified Surface Analysis Manual"(PDF).Hydrometeorological Prediction Center.Retrieved2010-12-17.
^"Mackerel sky".Weather Online.Retrieved21 November2013.
^Chris C. Park (2001).The environment: principles and applications.Psychology Press. p. 309.ISBN 978-0-415-21771-2.Retrieved2010-12-17.
^Donald, Ahrens, C. (2007).Meteorology today: an introduction to weather, climate, and the environment(8th ed.). Belmont, Calif.: Thomson/Brooks/Cole. pp. 298–300.ISBN 978-0495011620.OCLC 66911677.{{cite book}}:CS1 maint: multiple names: authors list (link)

External links

Warm Front: transition zone from cold air to warm air
Warm Fronts
A film clipAIR MASSES AND FRONTS - THE WARM FRONT (1962)is available for viewing at theInternet Archive

[DR-1] David Roth (2006-12-14)."Unified Surface Analysis Manual"(PDF).Hydrometeorological Prediction Center.Retrieved2010-12-17.

[Justice_Perrin-2] "Mackerel sky".Weather Online.Retrieved21 November2013.

[3] Chris C. Park (2001).The environment: principles and applications.Psychology Press. p. 309.ISBN 978-0-415-21771-2.Retrieved2010-12-17.

[4] Donald, Ahrens, C. (2007).Meteorology today: an introduction to weather, climate, and the environment(8th ed.). Belmont, Calif.: Thomson/Brooks/Cole. pp. 298–300.ISBN 978-0495011620.OCLC 66911677.{{cite book}}:CS1 maint: multiple names: authors list (link)

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