A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. For airmass in Astronomy, see Airmass. In Meteorology, an air mass is a large volume of Air that The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different A meteorological phenomenon is a Weather event which can be explained by the principles of Meteorology. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. A 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 The air masses separated by a front usually differ in temperature and humidity. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Severe weather is any destructive weather phenomenon. The term is usually used to refer to severe thunderstorms and related phenomena such as Tornados A squall line is a line of severe thunderstorms that can form along and/or ahead of a Cold front. A dry line, (also called dew point line, or Marfa front) is an important factor in Severe weather frequency in the Great Plains of Warm fronts are usually preceded by stratiform precipitation and fog. A warm front is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead Stratus means layer or blanket in Latin A Stratus cloud (St is a Cloud belonging to a class characterized by horizontal layering with a uniform base In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric Fog is a cloud that is in contact with the ground Stratus clouds are usually the only clouds that touch the ground 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. Occluded front symbolsvg|left|thumb|200px|Occluded front symbol]]An occluded front is formed during the process of Cyclogenesis when a Cold front overtakes a A geographical pole, or geographic pole, is either of two fixed points on the surface of a spinning body or Planet, at 90 degrees from the Equator, based 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. A mountain is a Landform that extends above the surrounding Terrain in a limited area with a peak [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. A stationary front is a boundary between two different Air masses neither of which is strong enough to replace the other This is most common over the open ocean.
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Bergeron classification of air masses
The Bergeron classification is the most widely accepted form of air mass classification. For airmass in Astronomy, see Airmass. In Meteorology, an air mass is a large volume of Air that 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. [3]
Surface weather analysis
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. A 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 A low pressure area, or " low " is a region where the Atmospheric pressure is lower in relation to the surrounding area A high pressure area (also called a high or high-pressure is a region where the Atmospheric pressure is greater than surrounding areas A weather map is a tool used to display information quickly showing the analysis of various meteorological quantities at various levels of the atmosphere [4] 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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Cloud cover (also known as cloudiness, cloudage or cloud amount) refers to the fraction of the Sky obscured by Clouds when observed The synoptic scale in Meteorology (also known as large scale or cyclonic scale) is a horizontal length scale of the order of 1000 kilometres (about 620 Surface weather analyses have special symbols which show frontal systems, cloud cover, precipitation, or other important information. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric 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. [4]
Front types
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. A cold front defined as the leading edge of a cooler and drier mass of air A cold front defined as the leading edge of a cooler and drier mass of air A contour line (also Level set, isopleth, isoline, isogram or isarithm) of a function of two A trough is an elongated region of relatively low Atmospheric pressure, often associated with fronts Unlike fronts there is not a universal symbol for 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. [2] Cold fronts come in association with a low pressure area. A low pressure area, or " low " is a region where the Atmospheric pressure is lower in relation to the surrounding 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. A shower (also called shower bath is a booth for washing usually in a Bathroom, having an overhead nozzle that sprays water down on the body This upward motion causes lowered pressure along the cold front. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface [2]
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 is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead A warm front is defined as the leading edge of an advancing mass of warm air it separates warm air from the colder air ahead 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. [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. Stratus means layer or blanket in Latin A Stratus cloud (St is a Cloud belonging to a class characterized by horizontal layering with a uniform base Fog can also occur preceding a warm frontal passage. Fog is a cloud that is in contact with the ground Stratus clouds are usually the only clouds that touch the ground 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. [2]
Occluded front
An occluded front is formed when a cold front overtakes a warm front. Occluded front symbolsvg|left|thumb|200px|Occluded front symbol]]An occluded front is formed during the process of Cyclogenesis when a Cold front overtakes a Occluded front symbolsvg|left|thumb|200px|Occluded front symbol]]An occluded front is formed during the process of Cyclogenesis when a Cold front overtakes a [5] The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point. [6] 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. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. [2] Occluded fronts usually form around mature low-pressure areas. [2]
Stationary front and shearline
A stationary front is a non-moving (or stalled) boundary between two air masses, neither of which is strong enough to replace the other. A stationary front is a boundary between two different Air masses neither of which is strong enough to replace the other A stationary front is a boundary between two different 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. [7] There is normally a broad temperature gradient behind the boundary with more widely spaced isotherm packing. In atmospheric sciences ( Meteorology, Climatology and related fields the temperature gradient (typically of air, more generally of any Fluid
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. [8] A shearline is depicted as a line of red dots and dashes. [2]
Dry line
A similar phenomenon to a weather front is the dry line, which is the boundary between air masses with significant moisture differences. A dry line, (also called dew point line, or Marfa front) is an important factor in Severe weather frequency in the Great Plains of A dry line, (also called dew point line, or Marfa front) is an important factor in Severe weather frequency in the Great Plains of When westerly winds aloft 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 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. Severe weather is any destructive weather phenomenon. The term is usually used to refer to severe thunderstorms and related phenomena such as Tornados [9] 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. [10]
The dry line may occur anywhere on earth in regions intermediate between desert areas and warm seas. A desert is a Landscape or region that receives very little precipitation. The southern plains west of the Mississippi River in the United States are a particularly favored location. The Mississippi River is the second longest River in the United States, with a length of from its source in Lake Itasca in Minnesota to The United States of America —commonly referred to as the 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. The National Weather Service ( NWS) once known as the Weather Bureau is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion. [11]
Squall line
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. A squall line is a line of severe thunderstorms that can form along and/or ahead of a Cold front. A mesoscale convective system ( MCS) is a complex of Thunderstorms that becomes organized on a scale larger than the individual thunderstorms and normally persists 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. [12] 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. [13]
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. A haboob is a type of intense sandstorm commonly observed in the Sahara desert (typically Sudan) as well as across the Arabian Peninsula throughout Kuwait and in [14] 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
Atlantic tropical waves develop from disturbances which drift off the continent of Africa onto the Atlantic Ocean. Tropical waves, or easterly waves, also known as African easterly waves in the Atlantic region are a type of Atmospheric trough, Tropical waves, or easterly waves, also known as African easterly waves in the Atlantic region are a type of Atmospheric trough, They are generated or enhanced by the African Easterly Jet. The African easterly jet is a region of the lower troposphere over West Africa where the seasonal mean wind speed is maximum and easterly The clockwise circulation of the large transoceanic high-pressure cell or anticyclone centered near the Azores islands moves easterly waves away from the coastal areas of Africa towards North America. A high pressure area (also called a high or high-pressure is a region where the Atmospheric pressure is greater than surrounding areas In Meteorology, an anticyclone (that is opposite to a Cyclone) is a Weather phenomenon in which there is a descending movement of the air and The Azores ( Açores ɐˈsoɾɨʃ or) is a Portuguese Archipelago in the Atlantic Ocean, about 1500 km (950 mi) from Tropical waves cause approximately 60% of Atlantic tropical cyclones and 85% of intense Atlantic hurricanes (Category 3 and greater). A tropical cyclone is a storm system characterized by a low pressure center and numerous Thunderstorms that produce strong winds and Flooding The Saffir-Simpson Hurricane Scale is a classification used for most Western Hemisphere Tropical cyclones that exceed the intensities of Tropical depressions [15][16]
Tropical cyclones sometimes degenerate back into a tropical wave. This normally occurs if upper-level wind shear is too strong. Wind shear, sometimes referred to as windshear or Wind gradient, is a difference in Wind speed and direction over a relatively 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 in 2003, where the original wave had winds of 45 mph (72 km/h) before developing a circulation. Hurricane Claudette was the third Tropical storm and first Hurricane of the 2003 Atlantic hurricane season. Tropical waves are depicted with a solid orange line on the U. S. National Weather Service Unified Surface Analysis. [2]
Precipitation produced
Fronts are the principal cause of significant weather. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric 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. Wind shear, sometimes referred to as windshear or Wind gradient, is a difference in Wind speed and direction over a relatively Jet streams are fast flowing relatively narrow air currents found at the Tropopause, the transition between the Troposphere (where temperature decreases A tornado is a violent rotating column of air which is in contact with both the surface of the earth and a Cumulonimbus cloud or in rare cases the base of a Cumulus [17]
In the warm season, lee troughs, breezes, outflow boundaries and occlusions can lead to convection if enough moisture is available. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. 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. [18] Although, not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted. [1]
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. In physics the Coriolis effect is an apparent deflection of moving objects when they are viewed from a Rotating frame of reference. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Frontal zones can be slowed down by geographic features like mountains and large bodies of warm water. [2]
See also
References
- ^ a b Author unknown. Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of Cyclones defined as synoptic scale low The older of the models of Extratropical cyclone development is known as the Norwegian Cyclone Model, developed during and shortly after World War I within the A 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 Lesson 7: Clouds and Precipitation. Self-published. Retrieved on 2007-04-29. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1429 - Joan of Arc arrives to relieve the Siege of Orleans.
- ^ a b c d e f g h i j k David Roth. Unified Surface Analysis Manual. Hydrometeorological Prediction Center. The Hydrometeorological Prediction Center (HPC is one of nine Service Centers under the umbrella of the National Centers for Environmental Prediction (NCEP operating under Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Glossary of Meteorology. Airmass Classification. Retrieved on 2008-05-22. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 334 BC - The Greek army of Alexander the Great defeats Darius III of Persia in the Battle of the Granicus.
- ^ a b Monmonier, Mark. Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago Press. Chicago: 1999.
- ^ Occluded Front. University of Illinois Department of Atmospheric Sciences. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ National Weather Service Office, Norman, Oklahoma. Triple Point. NOAA. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Stationary Front. University of Illinois Department of Atmospheric Sciences. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Shear Line. Glossary of Meteorology. American Meteorological Society. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Huaqing Cai. Dryline cross section. Retrieved on 2006-12-05. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 63 BC - Cicero reads the last of his Catiline Orations.
- ^ Lee Trough. Glossary of Meteorology. American Meteorological Society. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Dry Line: A Moisture Boundary. University of Illinois Department of Atmospheric Science. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Office of the Federal Coordinator for Meteorology. Chapter 2: Definitions. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Michael Branick. A Comprehensive Glossary of Weather. American Meteorological Society. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Western Region Climate Center. H. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. The Atlantic Oceanographic and Meteorological Laboratory (AOML is a laboratory in National Oceanic and Atmospheric Administration's Office of Oceanic and Atmospheric Research Frequently Asked Questions: What is an easterly wave?. NOAA. The National Oceanic and Atmospheric Administration ( NOAA) is a scientific agency within the United States Department of Commerce focused on the conditions of the Retrieved on 2006-07-25. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 285 - Diocletian appoints Maximian as Caesar, co-ruler
- ^ Avila, Lixion; Richard Pasch (March 1995). Dr Lixion Avila is a Weather forecaster with the National Hurricane Center (NHC "Atlantic tropical systems of 1993" (PDF). Monthly Weather Review 123 (3): 887–896. Monthly Weather Review is a publication of the American Meteorological Society. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Convection. Glossary of Meteorology. American Meteorological Society. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
- ^ Orographic Lifting. Glossary of Meteorology. American Meteorological Society. Retrieved on 2006-10-22. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 202 BC - Hannibal Barca, leader of the Carthaginians, is defeated by the Roman legions under Scipio Africanus
Bibliography
- (1999). Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago Press, Chicago.
External links
Dictionary
weather front
-noun
- (meteorology) The boundary between two masses of air of different density and/or temperature
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