Weather front

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, 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 thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform 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.[1]

Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts.[2] When a front becomes 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.

Unusually well defined warm front
Approaching weather fronts are often visible from the ground, but are not always as well defined as this.

Bergeron classification of air masses

The Bergeron classification is the most widely accepted form of air mass classification. Air mass classifications are indicated by 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 upward motion in the atmosphere). The third letter designates 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.[3] Fronts separate air masses of different types or origins, and are located along troughs of lower pressure.[4]

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. (cA is continental arctic; cP is continental polar; mP is maritime polar; cT is continental tropic; and mT is maritime tropic.)

Surface weather analysis

NWS weather fronts
Weather map symbols:
1. cold front;
2. warm front;
3. stationary front;
4. occluded front;
5. surface trough;
6. squall/shear line;
7. dry line;
8. tropical wave;
9. trowal

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-based weather stations.[5] Weather maps are created by plotting or tracing the values of relevant quantities such as sea-level pressure, temperature, and cloud cover onto a geographical map to help find synoptic scale features such as weather fronts. Surface weather analyses have special symbols which show frontal systems, cloud cover, 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. Low pressure also creates surface winds deriving from high pressure zones. 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.[5]


There are two different meanings used within meteorology to describe weather around a frontal zone. The term "anafront" describes boundaries which show instability, meaning air rises rapidly along and over the boundary to cause significant weather changes. A "katafront" is weaker, bringing smaller changes in temperature and moisture, as well as limited rainfall.[6]

Cold front

A cold front is located at the leading edge of the temperature drop off, which in an isotherm analysis shows up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts often bring heavy thunderstorms, rain, and hail. Cold fronts can produce sharper changes in weather and move up to twice as quickly as 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.[2] Cold fronts come in association with a low-pressure area. The concept of colder, dense air "wedging" under the less dense warmer air is often used to depict how air is lifted along a frontal boundary. The cold air wedging underneath warmer air creates the strongest winds just above the ground surface, a phenomenon often associated with property-damaging wind gusts. This lift would then form a narrow line of showers and thunderstorms if enough moisture were present. However, this concept isn't an accurate description of the physical processes;[7] upward motion is not produced because of warm air "ramping up" cold, dense air, rather, frontogenetical circulation is behind the upward forcing.

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 denser and harder to remove from the Earth's surface.[2]

This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly 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 semicircles pointing in the direction of travel.[2]

Occluded front

Occluded cyclone
Occluded front depiction for the Northern Hemisphere

An occluded front is formed when a cold front overtakes a warm front,[8] and usually forms around mature low-pressure areas.[2] The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point.[9] 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.[2]

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. Within the occlusion of the front, a circulation of air brings warm air upward and sends drafts of cold air downward, or vice versa depending on the occlusion the front is experiencing. Precipitations and clouds are associated with the trowal, the projection on the Earth's surface of the tongue of warm air aloft formed during the occlusion process of the depression.[10]

Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel.[2] The trowal is indicated by a series of blue and red junction lines.

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.[11] 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.[12] A shearline is depicted as a line of red dots and dashes.[2] Stationary fronts may bring snow or rain for a long period of time.

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 the westerlies increase on the north side of surface highs, areas of lowered 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 denser 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.[13] 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.[14]

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 an 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.[15]

Squall line

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 (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.[16] 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.[17]

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.[18] 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 BOUNDARY.

Precipitation produced

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 clouds" and tornadoes may occur.[19]

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.[20] Although, not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted.[1]


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.[2]

See also


  1. ^ a b Samuel Miller. "Lesson 7: Clouds and Precipitation". Archived from the original on 2005-01-11. Retrieved 2011-07-08.
  2. ^ a b c d e f g h i David Roth. "Unified Surface Analysis Manual" (PDF). Hydrometeorological Prediction Center. Retrieved 2006-10-22.
  3. ^ Glossary of Meteorology. Airmass Classification. Retrieved on 2008-05-22.
  4. ^ C. Donald Ahrens (2007). Meteorology today: an introduction to weather, climate, and the environment. Cengage Learning. p. 296. ISBN 978-0-495-01162-0.
  5. ^ a b Monmonier, Mark. Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago Press. Chicago: 1999.
  6. ^ Chris C. Park (2001). The environment: principles and applications. Psychology Press. p. 309. ISBN 978-0-415-21771-2.
  7. ^ "Overrunning". NWS Glossary. National Weather Service. Retrieved 2010-05-02.
  8. ^ "Occluded Front". University of Illinois Department of Atmospheric Sciences. Retrieved 2006-10-22.
  9. ^ National Weather Service Office, Norman, Oklahoma. "Triple Point". NOAA. Retrieved 2006-10-22.CS1 maint: Multiple names: authors list (link)
  10. ^ "Trowal". World Meteorological Organisation. Eumetcal. Archived from the original on 2014-03-31. Retrieved 2013-08-28.
  11. ^ Stationary Front. University of Illinois Department of Atmospheric Sciences. Retrieved on 2006-10-22.
  12. ^ "Shear Line". Glossary of Meteorology. American Meteorological Society. Archived from the original on 2007-03-14. Retrieved 2006-10-22.
  13. ^ Huaqing Cai. "Dryline cross section". Archived from the original on 2008-01-20. Retrieved 2006-12-05.
  14. ^ "Lee Trough". Glossary of Meteorology. American Meteorological Society. Archived from the original on 2011-09-19. Retrieved 2006-10-22.
  15. ^ "Dry Line: A Moisture Boundary". University of Illinois Department of Atmospheric Science. Retrieved 2006-10-22.
  16. ^ Office of the Federal Coordinator for Meteorology. "Chapter 2: Definitions" (PDF). Archived from the original (PDF) on 2009-05-06. Retrieved 2006-10-22.
  17. ^ Michael Branick. "A Comprehensive Glossary of Weather". American Meteorological Society. Retrieved 2006-10-22.
  18. ^ Western Region Climate Center. "H". Retrieved 2006-10-22.
  19. ^ "Convection". Glossary of Meteorology. American Meteorological Society. Retrieved 2006-10-22.
  20. ^ "Orographic Lifting". Glossary of Meteorology. American Meteorological Society. Retrieved 2006-10-22.


  • (1999). Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago Press, Chicago.

External links

Air mass

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of 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 (temperature and/or moisture) 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' characteristics, as well as modification.

Arctic front

The Arctic front is the semipermanent, semi-continuous weather front between the cold arctic air mass and the warmer air of the polar cell. It can also be defined as the southern boundary of the Arctic air mass. Mesoscale cyclones known as polar lows can form along the arctic front in the wake of extratropical cyclones. Arctic air masses in their wake are shallow with a deep layer of stable air above the shallow cold cool.

Blackwater Fire of 1937

On August 18, 1937, a lightning strike started the Blackwater Fire in Shoshone National Forest, approximately 35 miles (56 km) west of Cody, Wyoming, United States. Fifteen firefighters were killed by the forest fire when a dry weather front caused the winds to suddenly increase and change direction. The fire quickly spread into dense forest, creating spot fires that trapped some of the firefighters in a firestorm. Nine firefighters died during the fire and six more died shortly thereafter from severe burns and respiratory complications and another 38 firefighters were injured. It killed more professional wildland firefighters in the U.S. than any other in the 103 years between the Great Fire of 1910 and the Yarnell Hill Fire in 2013.

The Blackwater Fire consumed 1,700 acres (690 ha) of old-growth forest dominated by Douglas fir trees on the west slopes of Clayton Mountain. At the time the firestorm occurred, the temperatures were about 90 °F (32 °C) and the relative humidity was only 6 percent. Though most of the firefighters consisted of Civilian Conservation Corps (CCC) employees, they were led by more experienced United States Forest Service (USFS) fire managers. Firefighters in the first half of the 20th century used mostly hand tools to suppress wildfires, and all gear was carried by the firefighters or by pack animals. Weather forecasting and radio communication were generally poor or nonexistent.

Investigations and analysis of the event led the USFS to develop better ways to provide a more immediate response to combat fires; one of them was the development of the smokejumper program in 1939. Additionally, the Ten Standard Firefighting Orders, a standardized set of wildland firefighting principles, were developed in 1957. A year after the tragedy, survivors and their fellow employees constructed several memorials at the scene of the incident.

Cumulus congestus cloud

Cumulus congestus clouds, also known as towering cumulus, are a form of cumulus cloud that can be based in the low or middle height ranges. They achieve considerable vertical development in areas of deep, moist convection. They are an intermediate stage between cumulus mediocris and cumulonimbus.

East Asian rainy season

The East Asian rainy season, commonly called the plum rain (Chinese: 梅雨; pinyin: méiyǔ), is caused by precipitation along a persistent stationary front known as the Mei-Yu front for nearly two months during the late spring and early summer between eastern Russia, China, Korea, Taiwan, and Japan. The wet season ends during the summer when the subtropical ridge becomes strong enough to push this front north of the region.


A haboob (Arabic: هَبوب‎, romanized: habūb, lit. 'blasting/drifting') is a type of intense dust storm carried on an atmospheric gravity current, also known as a weather front. Haboobs occur regularly in dry land area regions throughout the world.

Ice crystals

Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust.


Khamsin, chamsin or hamsin (Arabic: خمسين‎ khamsīn, derived from the Arabic word for "fifty"), more commonly known in Egypt as khamaseen (Egyptian Arabic: خماسين‎ khamasīn, IPA: [xæmæˈsiːn]), is a dry, hot, sandy local wind affecting Egypt and Israel; similar winds, blowing in other parts of North Africa, the Arabian Peninsula and the entire Mediterranean basin, have different local names, such as bad-i-sad-o-bist roz in Iran and Afghanistan, haboob in the Sudan, aajej in southern Morocco, ghibli in Tunis, harmattan in the western Maghreb, africo in Italy, sirocco (derived from the Arabic sharkiyya, “easterly”) which blows in winter over much of the Middle East, and simoom.From the Arabic word for "fifty", these dry, sand-filled windstorms blow sporadically in Egypt over a fifty-day period in spring, hence the name. The term is also used in the southern Levant (Israel, Palestine, Jordan), where the phenomenon takes a partly different form and blows both during spring and autumn.When the storm passes over an area, lasting for several hours, it carries great quantities of sand and dust from the deserts, with a speed up to 140 kilometers per hour (87 mph; 76 knots), and the humidity in that area drops below 5%. Even in winter, the temperatures rise above 45° C (113° F) due to the storm. The sand storms are reported to have seriously impeded both Napoleon's military campaigns in Egypt as well as Allied-German fighting in North Africa in World War II.In the southern Levant it takes the shape of an oppressive weather front with hot temperatures, large quantities of dust impeding visibility, but no strong winds except during the night. In the Book of Exodus of the Hebrew Bible, the ruah kadim or "east wind" is the cause of the parting of the Red Sea (Exodus 14:21).

Lynmouth Flood

The Lynmouth Flood occurred on the night of the 15-16 August 1952, principally affecting the village of Lynmouth, in north Devon. A storm with heavy rainfall, combined with already saturated soil and flood debris, led to the flooding of the village and a total loss of 34 lives.

Occluded front

In meteorology, an occluded front is a weather front formed during the process of cyclogenesis, when a cold front overtakes a warm front. When this occurs, the warm air is separated (occluded) from the cyclone center at the Earth's surface. The point where the warm front and the occluded front meet (and consequently the nearest location of warm air to the center of the cyclone) is called the triple point.The trowal (short for TROugh of Warm air ALoft) is the projection on the Earth's surface of the trough of warm air aloft formed during the occlusion process of the depression.

Orographic lift

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.

Outline of meteorology

The following outline is provided as an overview of and topical guide to meteorology:

Meteorology – interdisciplinary scientific study of the atmosphere which explains and forecasts weather events. Meteorology has application in many diverse fields such as the military, energy production, transport, agriculture and construction.

Sea breeze

A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes are more localised than prevailing winds. Because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts after sunrise. By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea. Sea breezes and land breezes are both important factors in coastal regions' prevailing winds. The term offshore wind may refer to any wind over open water.

Wind farms are often situated near a coast to take advantage of the normal daily fluctuations of wind speed resulting from sea or land breezes. While many onshore wind farms and offshore wind farms do not rely on these winds, a nearshore wind farm is a type of offshore wind farm located on shallow coastal waters to take advantage of both sea and land breezes. (For practical reasons, other offshore wind farms are situated further out to sea and rely on prevailing winds rather than sea breezes.)


Stormfront may refer to:

A weather front

Stormfront Studios, a video game developer.

Stormfront (website), a white nationalist, white supremacist and neo-Nazi, Internet message forum; notable for being the first major hate website

Stormfront, a set from the Pokémon Trading Card Game

Stormfront, a character from comic book series, The Boys, by Garth Ennis.

Tropical Storm Bret (2011)

Tropical Storm Bret was the second named storm of the 2011 Atlantic hurricane season. Bret formed along the southwestern periphery of a weather front north of the Bahamas on July 17. At first, the storm moved little and gradually strengthened in response to favorable upper-level conditions, reaching peak sustained winds of 70 mph (110 km/h). Steering currents in the area subsequently became better established, and Bret turned toward the northeast only to encounter a substantial increase in vertical wind shear. Despite the shear, the storm maintained a well-defined wind circulation for several days, with intermittent bursts of thunderstorms near its center. By July 22, Bret had been devoid of strong thunderstorm activity for several hours, prompting the National Hurricane Center to discontinue public advisories when it was located about 375 miles (605 km) north of Bermuda.

Since Bret remained over the open Atlantic for most of its existence, its effects on land were limited. While moving little, the storm produced inclement weather and widespread cloudiness over much of the north-central Bahamas. Squalls off the eastern coast of Florida generated rough seas along coastlines, injuring a number of people. Although it stayed well offshore, the storm enhanced tropical moisture over Bermuda, causing beneficial rainfall in dry areas.

Trough (meteorology)

A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts. Troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line.

Unlike fronts, there is not a universal symbol for a trough on a weather chart. The weather charts in some countries or regions mark troughs by a line. In the United States, a trough may be marked as a dashed line or bold line. In the UK, Hong Kong and Fiji, it is represented by a bold line extended from a low pressure center or between two low pressure centers; in Macau and Australia, it is a dashed line. If they are not marked, troughs may still be identified as an extension of isobars away from a low pressure center.

Typhoon Megi (2010)

Typhoon Megi (pronounced [me̞.ɟi]), known in the Philippines as Super Typhoon Juan, was one of the most intense tropical cyclones on record. Megi, which means catfish in Korean (Hangul: 메기), was the only super typhoon in 2010. Early on October 18, Megi made its first landfall over Luzon. By passing Luzon, Megi weakened but gradually regained strength in the South China Sea, before weakening and losing its eye in the Taiwan Strait. Megi made its second landfall over Zhangpu in Fujian, China on October 23.Megi killed 31 people and caused $255.1 million (2010 USD) in damage over Luzon, making it one of the costliest typhoons in the Philippines. After moving to the South China Sea, the outflow of Megi and a weather front together brought torrential rainfall, caused $42.2 million (2010 USD) in damage and killed 38 people in Yilan, Taiwan, making Megi the deadliest typhoon of 2010s in Taiwan. Megi also caused $411.7 million (2010 USD) in damage over Fujian, China, although there were no deaths by the storm in the province.


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