Surface Weather Analysis

A surface weather analysis for the United States on October21, 2006.

Surface weather analysisFrom Wikipedia, the free encyclopedia (Redirected from Shear line (meteorology))

Surface weather analysis isa special type of weather mapthat provides a view ofweather elements over ageographical area at aspecified time based oninformation fromground-based weatherstations.[1] Weather maps arecreated by plotting or tracingthe values of relevantquantities such as sea levelpressure, temperature, andcloud cover onto ageographical map to help findsynoptic scale features suchas weather fronts.

The first weather maps in the19th century were drawn wellafter the fact to help devise a theory on storm systems.[2] After the advent of the telegraph,simultaneous surface weather observations became possible for the first time, andbeginning in the late 1840s, the Smithsonian Institution became the first organization todraw real-time surface analyses. Use of surface analyses began first in the United States,spreading worldwide during the 1870s. Use of the Norwegian cyclone model for frontalanalysis began in the late 1910s across Europe, with its use finally spreading to the UnitedStates during World War II.

Surface weather analyses have special symbols that show frontal systems, cloud cover,precipitation, or other important information. For example, an H may represent highpressure, implying good and fair weather. An L on the other hand may represent lowpressure, which frequently accompanies precipitation. Various symbols are used not justfor frontal zones and other surface boundaries on weather maps, but also to depict thepresent weather at various locations on the weather map. Areas of precipitation helpdetermine the frontal type and location.

Contents1 History of surface analysis2 Station model used on weather maps3 Synoptic scale features

3.1 Pressure centers3.1.1 Low pressure3.1.2 High pressure

Surface weather analysis - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Shear_line_(meteorology)

1 11 06: 49 29/ 03/ 2014PDF created with pdfFactory Pro trial version www.pdffactory.com

Surface analysis of GreatBlizzard of 1888 on March12, 1888 at 10 pm

3.2 Fronts3.2.1 Cold front3.2.2 Warm front3.2.3 Occluded front3.2.4 Stationary fronts and shearlines

4 Mesoscale features4.1 Dry line4.2 Outflow boundaries and squall lines4.3 Sea and land breeze fronts

5 See also6 References7 External links

History of surface analysisSee also: History of surface weather analysis

The use of weather charts in a modern sense began in themiddle portion of the 19th century in order to devise a theoryon storm systems.[3] The development of a telegraph networkby 1845 made it possible to gather weather information frommultiple distant locations quickly enough to preserve its valuefor real-time applications. The Smithsonian Institutiondeveloped its network of observers over much of the centraland eastern United States between the 1840s and 1860s onceJoseph Henry took the helm.[4] The U.S. Army Signal Corpsinherited this network between 1870 and 1874 by an act ofCongress, and expanded it to the west coast soon afterwards.

At first, all the data on the map was not taken from theseanalyses because of a lack of time standardization. The firstattempts at time standardization took hold in Great Britain by1855. The entire United States did not finally come under theinfluence of time zones until 1905, when Detroit finallyestablished standard time.[5] Other countries followed the lead of the United States intaking simultaneous weather observations, starting in 1873.[6] Other countries then beganpreparing surface analyses. The use of frontal zones on weather maps did not appear untilthe introduction of the Norwegian cyclone model in the late 1910s, despite Loomis' earlierattempt at a similar notion in 1841.[7] Since the leading edge of air mass changes boreresemblance to the military fronts of World War I, the term "front" came into use torepresent these lines.[8]

Despite the introduction of the Norwegian cyclone model just after World War I, the UnitedStates did not formally analyze fronts on surface analyses until late 1942, when the WBANAnalysis Center opened in downtown Washington, D.C..[9] The effort to automate mapplotting began in the United States in 1969,[10] with the process complete in the 1970s.Hong Kong completed their process of automated surface plotting by 1987.[11] By 1999,



Present weather symbols used onweather maps

Station model plotted on surfaceweather analyses

computer systems and software had finally becomesophisticated enough to allow for the ability to underlayon the same workstation satellite imagery, radarimagery, and model-derived fields such as atmosphericthickness and frontogenesis in combination withsurface observations to make for the best possiblesurface analysis. In the United States, thisdevelopment was achieved when Intergraphworkstations were replaced by n-AWIPSworkstations.[12] By 2001, the various surface analysesdone within the National Weather Service werecombined into the Unified Surface Analysis, which isissued every six hours and combines the analyses offour different centers.[13] Recent advances in both thefields of meteorology and geographic informationsystems have made it possible to devise finely tailored products that take us from thetraditional weather map into an entirely new realm. Weather information can quickly bematched to relevant geographical detail. For instance, icing conditions can be mappedonto the road network. This will likely continue to lead to changes in the way surfaceanalyses are created and displayed over the next several years.[14] The pressureNETproject is an ongoing attempt to gather surface pressure data using smartphones.

Station model used on weather mapsSee also: Station model

When analyzing a weather map, a station model isplotted at each point of observation. Within the stationmodel, the temperature, dewpoint, wind, sea levelpressure, pressure tendency, and ongoing weather areplotted.[15] The circle in the middle represents cloudcover. If completely filled in, it is overcast. If conditionsare completely clear, the circle is empty. If conditionsare partly cloudy, the circle is partially filled in.[16]Outside the United States, temperature and dewpointare plotted in degrees Celsius. Each full flag on theWind Barb represents 10 knots (19 km/h) of wind, eachhalf flag represents 5 knots (9 km/h). When windsreach 50 knots (93 km/h), a filled in triangle is used for each 50 knots (93 km/h) ofwind.[17] In the United States, rainfall plotted in the corner of the station model are inEnglish units, inches. The international standard rainfall measurement unit is themillimeter. Once a map has a field of station models plotted, the analyzing isobars (lines ofequal pressure), isallobars (lines of equal pressure change), isotherms (lines of equaltemperature), and isotachs (lines of equal wind speed) can be easily accomplished.[18]The abstract present weather symbols used on surface weather analyses for obstructionsto visibility, precipitation, and thunderstorms were devised to take up the least roompossible on weather maps.



Wind barb interpretation

Synoptic scale featuresSee also: Synoptic scale

A synoptic scale feature is one whose dimensions are large in scale, more than severalhundred kilometers in length.[19] Migratory pressure systems and frontal zones exist onthis scale.

Pressure centers

Centers of surface high- and low-pressure areas are foundwithin closed isobars on a surface weather analysis wherethey are the absolute maxima and minima in the pressurefield, and can tell a user in a glance what the general weatheris in their vicinity. Weather maps in English-speaking countrieswill depict their highs as Hs and lows as Ls,[20] while Spanish-speaking countries will depict their highs as As and lows asBs.[21]

Low pressure

Low-pressure systems, also known as cyclones, are located inminima in the pressure field. Rotation is inward andcounterclockwise in the northern hemisphere as opposed toinward and clockwise in the southern hemisphere due to the coriolis force. Weather isnormally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds,increased temperatures, and upward motion in the atmosphere, which leads to anincreased chance of precipitation. Polar lows can form over relatively mild ocean waterswhen cold air sweeps in from the ice cap, leading to upward motion and convection,usually in the form of snow. Tropical cyclones and winter storms are intense varieties oflow pressure. Over land, thermal lows are indicative of hot weather during the summer.[22]

High pressure

High-pressure systems, also known as anticyclones, rotate outward and clockwise in thenorthern hemisphere as opposed to outward and counterclockwise in the southernhemisphere. Under surface highs, sinking motion leads to skies that are clearer, winds thatare lighter, and there is a reduced chance of precipitation.[23] There is normally a greaterrange between high and low temperature due to the drier air mass present. If highpressure persists, air pollution will build up due to pollutants trapped near the surfacecaused by the subsiding motion associated with the high.[24]

Fronts

Main article: Weather fronts

Fronts in meteorology are the leading edges of air masses with different density (e.g., airtemperature and/or humidity). When a front passes over an area, it is marked by changes



Occluded cyclone example. Thetriple point is the intersection ofthe cold, warm, and occludedfronts.

Illustration clouds overriding awarm front

in temperature, moisture, wind speed and direction,atmospheric pressure, and often a change in theprecipitation pattern. Cold fronts are closely associatedwith low pressure systems, normally lying at theleading edge of high-pressure systems and, in thecase of the polar front, at approximately theequatorward edge of the high-level polar jet. Fronts areguided by winds aloft, but they normally move at lesserspeeds. In the northern hemisphere, they usually travelfrom west to east (though they can move in anorth-south direction as well). Movement is due to thepressure gradient force (horizontal differences inatmospheric pressure) and the Coriolis effect, causedby the earth spinning about its axis. Frontal zones canbe contorted by geographic features like mountainsand large bodies of water.[13]

Cold front

Main article: Cold front

A cold front's location is at the leading edge of the temperature drop off, which in anisotherm analysis shows up as the leading edge of the isotherm gradient, and it normallylies within a sharp surface trough. Cold fronts can move up to twice as fast as warm frontsand produce sharper changes in weather, since cold air is denser than warm air andrapidly replaces the warm air preceding the boundary. Cold fronts are typicallyaccompanied by a narrow band of showers and thunderstorms. On weather maps, thesurface position of the cold front is marked with the symbol of a blue line oftriangles/spikes (pips) pointing in the direction of travel, and it is placed at the leadingedge of the cooler air mass.[13]

Warm front

Main article: Warm front

Warm fronts are at the trailing edge of the temperatureincrease, which is located on the equatorward edge ofthe gradient in isotherms, and lie within broadertroughs of low pressure than cold fronts. Warm frontsmove more slowly than the cold front that usuallyfollows because cold air is denser, and harder todisplace from the Earth's surface. This also forcestemperature differences across warm fronts to bebroader in scale. Clouds ahead of the warm front aremostly stratiform and rainfall gradually increases as thefront approaches. Fog can also occur preceding awarm frontal passage. Clearing and warming is usuallyrapid after frontal passage. If the warm air mass isunstable, thunderstorms may be embedded among the stratiform clouds ahead of thefront, and after frontal passage, thundershowers may continue. On weather maps, the



A guide to the symbols for weatherfronts that may be found on aweather map:1. cold front2. warm front3. stationary front4. occluded front5. surface trough6. squall line7. dry line8. tropical wave9. Trowal

surface location of a warm front is marked with a red line of half circles pointing in thedirection of travel.[13]

Occluded front

Main article: Occluded front

An occluded front is formed during the process ofcyclogenesis when a cold front overtakes a warmfront.[25] The cold and warm fronts curve naturallypoleward into the point of occlusion, which is alsoknown as the triple point in meteorology.[26] It lieswithin a sharp trough, but the air mass behind theboundary can be either warm or cold. In a coldocclusion, the air mass overtaking the warm front iscooler than the cool air ahead of the warm front, andplows under both air masses. In a warm occlusion, theair mass overtaking the warm front is not as cool as thecold air ahead of the warm front, and rides over thecolder air mass while lifting the warm air. A wide varietyof weather can be found along an occluded front, withthunderstorms possible, but usually their passage isassociated with a drying of the air mass. Occludedfronts are indicated on a weather map by a purple linewith alternating half-circles and triangles pointing indirection of travel.[13] Occluded fronts usually formaround mature low pressure areas.

The trowal is the projection on the Earth's surface of a tongue of warm air aloft, such asmay be formed during the occlusion process of a depression.[27]

Stationary fronts and shearlines

Main article: Stationary front

A stationary front is a non-moving boundary between two different air masses, neither ofwhich is strong enough to replace the other. They tend to remain in the same area for longperiods of time, usually moving in waves.[28] There is normally a broad temperaturegradient behind the boundary with more widely spaced isotherm packing. A wide variety ofweather can be found along a stationary front, but usually clouds and prolongedprecipitation are found there. Stationary fronts will either dissipate after several days ordevolve into shear lines, but can change into a cold or warm front if conditions aloftchange. Stationary fronts are marked on weather maps with alternating red half-circles andblue spikes pointing in opposite directions, indicating no significant movement.

When stationary fronts become smaller in scale, degenerating to a narrow zone wherewind direction changes over a short distance, they become known as shear lines.[29] If theshear line becomes active with thunderstorms, it may support formation of a tropical stormor a regeneration of the feature back into a stationary front. A shear line is depicted as a



A shelf cloud such as this one canbe a sign that a squall is imminent

line of red dots and dashes.[13]

Mesoscale featuresSee also: Mesoscale Convective System

Mesoscale features are smaller than synoptic scale systems like fronts, but larger thanstorm-scale systems like thunderstorms. Horizontal dimensions generally range from overten kilometres to several hundred kilometres.[30]

Dry line

The dry line is the boundary between dry and moist air masses east of mountain rangeswith similar orientation to the Rockies, depicted at the leading edge of the dew point, ormoisture, gradient. Near the surface, warm moist air that is denser than dry air of greatertemperature wedges under the drier air like a cold front.[31] When the warm moist airwedged under the drier mass heats up it becomes less dense than the drier air above andit begins to rise and sometimes forms thunderstorms.[32] At higher altitudes, the warmmoist air is less dense than the cooler, drier air and the boundary slope reverses. In thevicinity of the reversal aloft, severe weather is possible, especially when a triple point isformed with a cold front.

During daylight hours, drier air from aloft drifts down to the surface, causing an apparentmovement of the dryline eastward. At night, the boundary reverts to the west as there is nolonger any sunshine to help mix the lower atmosphere.[33] If enough moisture convergesupon the dryline, it can be the focus of afternoon and evening thunderstorms.[34] A dry lineis depicted on United States surface analyses as a brown line with scallops, or bumps,facing into the moist sector. Dry lines are one of the few surface fronts where the specialshapes along the drawn boundary do not necessarily reflect the boundary's direction ofmotion.[35]

Outflow boundaries and squall lines

Organized areas of thunderstorm activity not onlyreinforce pre-existing frontal zones, but they can outruncold fronts. This outrunning occurs in a pattern wherethe upper level jet splits into two streams. The resultantmesoscale convective system (MCS) forms at the pointof the upper level split in the wind pattern in the area ofbest low level inflow. The convection then moves eastand equatorward into the warm sector, parallel tolow-level thickness lines. When the convection isstrong and linear or curved, the MCS is called a squallline, with the feature placed at the leading edge of the significant wind shift and pressurerise.[36] Even weaker and less organized areas of thunderstorms will lead to locally coolerair and higher pressures, and outflow boundaries exist ahead of this type of activity,"SQLN" or "SQUALL LINE", while outflow boundaries are depicted as troughs with a labelof "OUTFLOW BOUNDARY" or "OUTFLOW BNDRY".



Idealized circulation patternassociated with a sea breeze

Sea and land breeze fronts

Sea breeze fronts occur mainly on sunny days whenthe landmass warms up above the water temperature.Similar boundaries form downwind on lakes and riversduring the day, as well as offshore landmasses atnight. Since the specific heat of water is so high, thereis little diurnal change in bodies of water, even on thesunniest days. The water temperature varies less than1 C (1 to 2 F). By contrast, the land, with a lowerspecific heat, can vary several degrees in a matter ofhours.[37]

During the afternoon, air pressure decreases over theland as temperature rises. The relatively cooler air overthe sea rushes in to fill the gap. The result is a relatively cool onshore wind. This processusually reverses at night where the water temperature is higher relative to the landmass,leading to an offshore land breeze. However, if water temperatures are colder than theland at night, the sea breeze may continue, only somewhat abated. This is typically thecase along the California coast, for example.

If enough moisture exists, thunderstorms can form along sea breeze fronts that then cansend out outflow boundaries. This causes chaotic wind/pressure regimes if the steeringflow is light. Like all other surface features, sea breeze fronts lie inside troughs of lowpressure.

See alsoAmerican Practical NavigatorCyclogenesisExtratropical cycloneSynoptic scaleWeather frontsWeather mapOutline of meteorology

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External links"The Mid-Latitude Cyclone" (http://www.physicalgeography.net/fundamentals/7s.html)Norwegian Cyclone Model NWS (http://www.srh.weather.gov/srh/jetstream/synoptic/cyclone.htm)Unified Surface Analysis Manual NWS (http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf)Unified Surface Analysis NWS (http://www.opc.ncep.noaa.gov/index.shtml)Glossary of Meteorology (http://amsglossary.allenpress.com/glossary/)Cold Front Page (http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/cfrnt/def.rxml)Diana : A Free Meteorological Visualisation Tool (http://met.no/diana/)

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