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In meteorology, the synoptic scale (also called the large scale or cyclonic scale) is a horizontal length scale of the order of or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e.g. extratropical cyclones). Most high- and low-pressure areas seen on (such as surface weather analyses) are synoptic-scale systems, driven by the location of 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 is derived from the Ancient Greek word (), meaning "seen together".
The Navier–Stokes equations applied to atmospheric motion can be simplified by scale analysis in the synoptic scale. It can be shown that the main terms in horizontal equations are Coriolis force and pressure gradient terms; therefore, one can use geostrophic wind. In vertical coordinates, the momentum equation simplifies to the hydrostatic equilibrium equation.
The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems.Eric R. Miller. American Pioneers in Meteorology. Retrieved on 2007-04-18. After the advent of the telegraph, simultaneous surface weather observations became possible for the first time. Beginning in the late 1840s, the Smithsonian 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 the Norwegian cyclone model for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during World War II.
Surface weather analyses have special symbols which show frontal systems, cloud cover, precipitation, or other important information. For example, an H represents high pressure, implying good and fair weather. An L represents 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. Areas of precipitation help determine the frontal type and location. Mesoscale systems and boundaries such as , outflow boundaries and are also analyzed on surface weather analyses. Isobars are commonly used to place surface boundaries from the horse latitudes poleward, while streamline analyses are used in the tropics.Bureau of Meteorology. The Weather Map. Retrieved on 10 May 2007.
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" where extratropical transition has occurred, but are often described as "depressions" or "lows" by weather forecasters and the public. These are the everyday phenomena that, along with , drive the weather over much of the Earth.
Although extratropical cyclones are almost always classified as baroclinic since they form along zones of temperature and dew point gradient within the westerlies, they can sometimes become barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.Ryan N. Maue. CHAPTER 3: CYCLONE PARADIGMS AND EXTRATROPICAL TRANSITION CONCEPTUALIZATIONS. Retrieved on 15 June 2008. 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.
Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses.CBC News (2009). Blame Yukon: Arctic air mass chills rest of North America. Canadian Broadcasting Centre. Retrieved on 2009-02-16. The low, sharp inversion can lead to areas of persistent stratocumulus or stratus 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 as confluence, 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.Glossary of Meteorology (2009). Level of nondivergence. American Meteorological Society. Retrieved on 2009-02-17.Konstantin Matchev (2009). Middle-Latitude Cyclones - II. University of Florida. Retrieved on 2009-02-16. High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English,Keith C. Heidorn (2005). Weather's Highs and Lows: Part 1 The High. The Weather Doctor. Retrieved on 2009-02-16. or A in Spanish,Instituto Nacional de Meteorologia. Meteorologia del Aeropuerto de la Palma. Retrieved on 2007-05-05. because alta is the Spanish word for high, within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.Glossary of Meteorology (2009). High. American Meteorological Society. Retrieved on 2009-02-16.
Cold fronts and 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. and warm bodies of water can slow the movement of fronts. When a front becomes stationary front, 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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