Cold front

Last updated November 15, 2024

A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern Hemisphere, to the east in the Southern), at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak,^{[ clarification needed ]} a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.

Development of cold fronts

A cold front occurs when a mass of comparatively colder air moves into where warmer air is present. The drier, colder air forms a steeply sloping boundary under the warmer, moister air at the surface and lifts that air. This often causes cloud formations with a strong vertical development, which may manifest as a line of showers and thunderstorms when enough moisture is present.^[1] On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough.^[2] Cold fronts move faster than warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary.^[3]

In the northern hemisphere, a cold front usually causes a shift of wind from southwest to northwest clockwise, also known as veering, and in the southern hemisphere a shift from northwest to southwest (counterclockwise, backing). Atmospheric pressure steadily decreases with the approach of a cold front. With frontal passage, the pressure rises sharply and then stabilizes. Normally, cold fronts can be marked by these characteristics:^[4]^[1]

Weather phenomenon	Prior to the passing of the front	While the front is passing	After the passing of the front
Temperature	Warm	Cooling suddenly	Steadily cooling
Atmospheric pressure	Decreasing steadily	Lowest, then sudden increase	Increasing steadily
Winds	Southwest to southeast (northern hemisphere) Northwest to northeast (southern hemisphere)	Gusty; shifting	North to west, usually northwest (northern hemisphere) South to west, usually southwest (southern hemisphere)
Precipitation/conditions*	Light patchy rain can be produced by stratocumulus or stratus in the warm sector. In summer, sometimes thunderstorms if a preceding squall line is present. In winter snow squalls or showers may occur.^[5]	Prolonged rain (nimbostratus) or thunderstorms (cumulonimbus): depends on conditions.	Showers, then clearing
Clouds*	Often preceded by cirrus, cirrostratus then altostratus like a warm front (but usually with smaller amounts of these clouds). Areas of cirrocumulus and altocumulus within cirrostratus and altostratus more commonly seen than at a warm front. Larger cumulus clouds under the higher cloud types than at a warm front, where stratocumulus and cumulus humilis usually occur. Some of these cumulus clouds may produce showers ahead of the front.	Cumulonimbus and cumulus congestus producing frequent showers, with a sheet of upper altostratus, through which the sun can sometimes be seen. Less commonly nimbostratus occurs with continuous rain.	Patchy altocumulus or stratocumulus and higher cirrus clouds along with fast moving stratus fractus then eventually scattered cumulus and sometimes cumulonimbus.
Visibility*	Fair to poor in haze	Poor, but improving	Good, except in showers
Dew Point	High, steady	Sudden drop	Falling

*provided there is sufficient moisture.

Clouds

If the cold front is highly unstable, cumulonimbus clouds producing thunderstorms commonly form along the front. Anvil cirrus clouds may spread a considerable distance downwind from the thunderstorms.^[6] The other cloud types associated with a cold front depend on atmospheric conditions such as airmass stability and wind shear.^[7] As the front approaches, middle-étage gives way to altostratus and low-level stratocumulus with intermittent light precipitation if the warm airmass being displaced by the cold front is mostly stable. With significant airmass instability, vertically developed cumulus or cumulonimbus with showers and thunderstorms will form along the front.

After the passage of the cold front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called cloud streets, may persist if the airmass behind the front remains humid from a source of moisture.^[8] Small and unchanging amounts of cumulus or cirrus clouds in an otherwise clear sky are usually indications of continuing fair weather as long as the barometric pressure remains comparatively high.

Altocumulus undulatus clouds several hours after a cold front AltocumulusUndulatusClouds.png — Altocumulus undulatus clouds several hours after a cold front

Precipitation

Strong thunderstorm associated with a cold front in Northern Mexico

A cold front commonly brings a narrow band of precipitation that follows along the leading edge of the cold front. These bands of precipitation are often very strong,^[9] and can bring severe thunderstorms, hailstorms,^[10] snow squalls,^[5] and/or tornadoes. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient is higher than normal. During the winter months, cold fronts sometimes come through an area with little or no precipitation. Wider rain bands can occur behind cold fronts which tend to have more stratiform, and less convective, precipitation.^[11] These rainstorms sometimes bring flooding, and can move very slowly when the storm steering it is strong and embedded within a meridional flow pattern (with more pole to equator motion rather than west to east motion). In the winter, cold fronts can bring cold spells, and occasionally snow. In the spring or summer in temperate latitudes, hail may occasionally fall along with the rain. If moisture is not sufficient, such as when a system has previously moved across a mountain barrier, cold fronts can pass without cloudiness.

Frontogenetical circulation

Frontogenesis is the process of creating or steepening the temperature gradient of a front. During this process the atmosphere reacts in an attempt to restore balance, the consequence is a circular motion along the front where air is being lifted up, along the cold front and dropping downward, behind the frontal boundary. This is the actual force of upward motion along a front that is responsible for clouds and precipitation.

As the temperature gradient steepens during frontogenesis, the thermal wind becomes imbalanced. To maintain balance, the geostrophic wind aloft and below adjust, such that regions of divergence/convergence form. Mass continuity would require a vertical transport of air along the cold front where there is divergence (lowered pressure). Although this circulation is described by a series of processes, they are actually occurring at the same time, observable along the front as a thermally direct circulation. There are several factors that influence the final shape and tilt of the circulation around the front, ultimately determining the kind and location of clouds and precipitation.^[7]^[12]

Temperature changes

Cold fronts are the leading edge of cooler air masses, hence the name "cold front". They have stronger temperature changes during the fall (autumn) and spring, and during the middle of winter. Temperature changes associated with cold fronts can be as much as 30 °C (54 °F). When cold fronts come through, there is usually a quick, yet strong gust of wind, that shows that the cold front is passing. In surface weather observations, a remark known as FROPA is coded when this occurs.^[13] The effects from a cold front can last from hours to days. The air behind the front is cooler than the air it is replacing and the warm air is forced to rise, so it cools. As the cooler air cannot hold as much moisture as warm air, clouds form and rain or snow occur.^[4]

Characteristics of boundaries around an extratropical cyclone

Cold fronts form when a cooler air mass moves into an area of warmer air in the wake of a developing extratropical cyclone. The warmer air interacts with the cooler air mass along the boundary, and usually produces precipitation. Cold fronts often follow a warm front or squall line. Very commonly, cold fronts have a warm front ahead but with a perpendicular orientation. In areas where cold fronts catch up to the warm front, the occluded front develops. Occluded fronts have an area of warm air aloft. When such a feature forms poleward of an extratropical cyclone, it is known as a trowal , which is short for TRough Of Warm Air aLoft.^[14] A cold front is re-designated a warm front if it begins to retreat ahead of the next extratropical cyclone along the frontal boundary, and called a stationary front if it stalls. An especially sharp type of cold front, easily discernable in satellite images, is the Narrow Cold Frontal Rainband.

Related Research Articles

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

<span class="mw-page-title-main">Air mass</span> Volume of air defined by its temperature and water vapor content

In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry, while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass's characteristics, as well as modification.

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".

A warm front is a 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 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. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly 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 weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.

A stationary front is a weather front or transition zone between two air masses when each air mass is advancing into the other at speeds less than 5 knots at the ground surface. These fronts are typically depicted on weather maps as a solid line with alternating blue spikes and red domes.

A dry line is a line across a continent that separates moist air and dry air. One of the most prominent examples of such a separation occurs in central North America, especially Texas, Oklahoma, and Kansas, where the moist air from the Gulf of Mexico meets dry air from the desert south-western states. The dry line is an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the High Plains states in the warm sector of an extratropical cyclone and stretches into the Canadian Prairies during the spring and early summer. The dry line is also important for severe convective storms in other regions of the world, such as northern India and Southern Africa. In general, thunderstorms and other forms of severe weather occur on the moist side of the dryline.

<span class="mw-page-title-main">Cyclogenesis</span> The development or strengthening of cyclonic circulation in the atmosphere

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands in tropical cyclones can be either stratiform or convective and are curved in shape. They consist of showers and thunderstorms, and along with the eyewall and the eye, they make up a tropical cyclone. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

A snow squall, or snowsquall, is a sudden moderately heavy snowfall with blowing snow and strong, gusty surface winds. It is often referred to as a whiteout and is similar to a blizzard but is localized in time or in location and snow accumulations may or may not be significant.

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to severe gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain.

Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.

Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.

<span class="mw-page-title-main">Col (meteorology)</span>

A col, also called saddle point or neutral point, is in meteorology, the point of intersection of a trough and a ridge in the pressure pattern of a weather map. It takes the form of a saddle where the air pressure is relatively higher than that of the low-pressure regions, but lower than that of the anticyclonic zones.

This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.

References

1 2 Whiteman, C. David (2000). Mountain meteorology : fundamentals and applications. New York: Oxford University Press. pp. 77–79. ISBN 978-0-19-803044-7. OCLC 428735924.
↑ David Roth (2006-12-14). "Unified Surface Analysis Manual" (PDF). Hydrometeorological Prediction Center . Retrieved 2012-01-09.
↑ Paul M. Markowski; Yvette P. Richardson (2011-09-20). Mesoscale Meteorology in Midlatitudes. John Wiley and Sons. p. 120. ISBN 978-1-119-96667-8 . Retrieved 2012-01-09.
1 2 "Cold Front: transition zone from warm air to cold air". ww2010.atmos.uiuc.edu. Retrieved 2019-11-09.
1 2 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)
↑ Lee M. Grenci; Jon M. Nese (2001). A World of Weather: Fundamentals of Meteorology: A Text / Laboratory Manual (3rd ed.). Kendall/Hunt Publishing Company. pp. 207–212. ISBN 978-0-7872-7716-1. OCLC 51160155.
1 2 Holton, James R. (2004). An Introduction to Dynamic Meteorology. Academic Press. p. 277. ISBN 978-0-12-354015-7.
↑ Weston, K. J. (1980). "An observational study of convective cloud streets". Tellus. 32 (5): 433–438. Bibcode:1980Tell...32..433W. doi:10.1111/j.2153-3490.1980.tb00970.x.
↑ Glossary of Meteorology (2009). Prefrontal squall line. Archived 2007-08-17 at the Wayback Machine Retrieved on 2008-12-24.
↑ Schemm, S.; L. Nisi, A. Martinov; D. Leuenberg & O. Martius (2016). "On the link between cold fronts and hail in Switzerland". Atmospheric Science Letters. 17 (5): 315–325. Bibcode:2016AtScL..17..315S. doi: 10.1002/asl.660 . hdl: 1956/12444 .
↑ K. A. Browning and Robert J. Gurney (1999). Global Energy and Water Cycles. Retrieved on 2008-12-26.
↑ Carlson, Toby N. (1991). Mid-latitude Weather Systems. HarperCollins Academic. p. 435. ISBN 978-0-04-551115-0.
↑ Nav Canada (January 2005). Aviation Weather Services Guide. p. 36.
↑ St. Louis University (2003-08-04). "What is a TROWAL? via the Internet Wayback Machine". Archived from the original on 2006-09-16. Retrieved 2006-11-02.

External links

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[:2-1] 1 2 Whiteman, C. David (2000). Mountain meteorology : fundamentals and applications. New York: Oxford University Press. pp. 77–79. ISBN 978-0-19-803044-7. OCLC 428735924.

[DR-2] David Roth (2006-12-14). "Unified Surface Analysis Manual" (PDF). Hydrometeorological Prediction Center . Retrieved 2012-01-09.

[3] Paul M. Markowski; Yvette P. Richardson (2011-09-20). Mesoscale Meteorology in Midlatitudes. John Wiley and Sons. p. 120. ISBN 978-1-119-96667-8 . Retrieved 2012-01-09.

[:1-4] 1 2 "Cold Front: transition zone from warm air to cold air". ww2010.atmos.uiuc.edu. Retrieved 2019-11-09.

[:0-5] 1 2 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)

[G&N:207-212-6] Lee M. Grenci; Jon M. Nese (2001). A World of Weather: Fundamentals of Meteorology: A Text / Laboratory Manual (3rd ed.). Kendall/Hunt Publishing Company. pp. 207–212. ISBN 978-0-7872-7716-1. OCLC 51160155.

[Holt-7] 1 2 Holton, James R. (2004). An Introduction to Dynamic Meteorology. Academic Press. p. 277. ISBN 978-0-12-354015-7.

[Weston437-438-8] Weston, K. J. (1980). "An observational study of convective cloud streets". Tellus. 32 (5): 433–438. Bibcode:1980Tell...32..433W. doi:10.1111/j.2153-3490.1980.tb00970.x.

[9] Glossary of Meteorology (2009). Prefrontal squall line. Archived 2007-08-17 at the Wayback Machine Retrieved on 2008-12-24.

[10] Schemm, S.; L. Nisi, A. Martinov; D. Leuenberg & O. Martius (2016). "On the link between cold fronts and hail in Switzerland". Atmospheric Science Letters. 17 (5): 315–325. Bibcode:2016AtScL..17..315S. doi: 10.1002/asl.660 . hdl: 1956/12444 .

[11] K. A. Browning and Robert J. Gurney (1999). Global Energy and Water Cycles. Retrieved on 2008-12-26.

[Carl-12] Carlson, Toby N. (1991). Mid-latitude Weather Systems. HarperCollins Academic. p. 435. ISBN 978-0-04-551115-0.

[13] Nav Canada (January 2005). Aviation Weather Services Guide. p. 36.

[TROW-14] St. Louis University (2003-08-04). "What is a TROWAL? via the Internet Wayback Machine". Archived from the original on 2006-09-16. Retrieved 2006-11-02.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]