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| Meteorology |
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| Climatology |
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| Glossaries |
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Inmeteorology, thesynoptic scale (also called thelarge scale orcyclonic scale) is ahorizontallength scale of theorder of 1,000 km (620 mi) or more.[1] This corresponds to a horizontal scale typical ofmid-latitudedepressions (e.g.extratropical cyclones). Mosthigh- andlow-pressure areas seen onweather maps (such assurface weather analyses) are synoptic-scale systems, driven by the location ofRossby 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, whilehigh-pressure areas form on the back edge of the trough. Mostprecipitation areas occur near frontal zones.
TheNavier–Stokes equations applied to atmospheric motion can be simplified byscale analysis in the synoptic scale. It can be shown that the main terms in horizontal equations areCoriolis force andpressure gradient terms; therefore, one can usegeostrophic approximation. In vertical coordinates, the momentum equation simplifies to thehydrostatic equilibrium equation.
The wordsynoptic is derived from theAncient Greek wordσυνοπτικός (sunoptikós), meaning "seen together".
Asurface weather analysis is a special type ofweather map that provides a view ofweather elements over a geographical area at a specified time based on information from ground-based weather stations.[2] Weather maps are created by plotting or tracing the values of relevant quantities such assea level pressure,temperature, andcloud cover onto ageographical map to help findsynoptic scale features such asweather fronts.
The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems.[3] After the advent of thetelegraph, simultaneoussurface weather observations became possible for the first time. Beginning in the late 1840s, theSmithsonian Institution became the first organization to draw real-time surface analyses. Use of surface analyses began first in the United States, spreading worldwide during the 1870s. Use of theNorwegian cyclone model for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States duringWorld War II.
Surface weather analyses have special symbols which show frontal systems, cloud cover,precipitation, or other important information. For example, anH representshigh pressure, implying good and fair weather. AnL representslow 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. Areas of precipitation help determine the frontal type and location. Mesoscale systems and boundaries such astropical cyclones, outflow boundaries andsquall lines are also analyzed on surface weather analyses. Isobars are commonly used to place surface boundaries from thehorse latitudes poleward, while streamline analyses are used in the tropics.[4]
An extratropical cyclone is a synoptic scalelow-pressure weather system that has neithertropical norpolar characteristics, being connected withfronts and horizontalgradients intemperature anddew point otherwise known as "baroclinic zones".[5]
The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" whereextratropical transition has occurred,[5][6] but are often described as "depressions" or "lows" by weather forecasters and the public. These are the everyday phenomena that, along withanticyclones, drive the weather over much of the Earth.
Although extratropical cyclones are almost always classified asbaroclinic since they form along zones of temperature and dew point gradient within thewesterlies, they can sometimes becomebarotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.[7] An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core.[8]
High-pressure systems are frequently associated with light winds at the surface andsubsidence through the lower portion of thetroposphere. Subsidence will generally dry out an air mass byadiabatic, or compressional, heating.[9] Thus, high pressure typically brings clear skies.[10] During the day, since no clouds are present to reflect sunlight, there is more incoming shortwavesolar radiation and temperatures rise. At night, the absence of clouds means thatoutgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving coolerdiurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespreadhaze.[11] If the low levelrelative humidity rises towards 100 percent overnight,fog can form.[12]
Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses.[13] The low, sharpinversion can lead to areas of persistentstratocumulus orstratus cloud, colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming.
On weather maps, these areas show converging winds (isotachs), also known asconfluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere.[14][15] High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English,[16] or A in Spanish,[17] because alta is the Spanish word for high, within theisobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.[18]
Aweather front is a boundary separating twomasses of air of differentdensities, and is the principal cause ofmeteorological phenomena. Insurface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front. The air masses separated by a front usually differ intemperature andhumidity.Cold fronts may feature narrow bands ofthunderstorms andsevere weather, and may on occasion be preceded bysquall lines ordry lines.Warm fronts are usually preceded bystratiformprecipitation andfog. 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.[19]
Cold fronts andoccluded fronts generally move from west to east, while warm fronts movepoleward. 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.[20] When a front becomesstationary, 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.
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