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 miles) or more.^[1] This corresponds to a horizontal scale typical of mid-latitude depressions. Most high and low pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low pressure areas form on the leading edge of a trough within the Rossby wave pattern, while surface highs form on the back edge of the trough. The word synoptic is derived from the Greek word sunoptikos 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 main terms in horizontal equations are Coriolis force and pressure gradient terms; therefore, one can use geostrophic approximation. In vertical coordinates the momentum equation simplifies to the hydrostatic equilibrium equation. Extratropical cyclones in particular are driven by large scale waves at upper levels of the troposphere known as rossby waves.

[edit] Atmospheric Rossby waves

Meanders of the northern hemisphere's jet stream developing (a, b) and finally detaching a "drop" of cold air (c). Orange: warmer masses of air; pink: jet stream.

Rossby waves in the atmosphere are easy to observe as (usually 4-6) large-scale meanders of the jet stream across the Northern or Southern Hemispheres. When these loops become very pronounced, they detach the masses of cold, or warm, air that become cyclones and anticyclones and are responsible for day-to-day weather patterns at mid-latitudes.

The wave speed is given by

where c is the wave speed, u is the mean westerly flow, β is the Rossby parameter, and k is the total wavenumber.

Furthermore, the Rossby parameter is defined:

φ is the latitude, ω is the angular speed of the Earth's rotation, and a is the mean radius of the Earth.

[edit] Surface weather analysis

A surface weather analysis for the United States on October 21, 2006.

A surface weather analysis is a type of weather map that depicts positions for high and low pressure areas, as well as various types of synoptic scale systems such as frontal zones. Mesoscale boundaries such as tropical cyclones, outflow boundaries and squall lines also are 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.^[2]

[edit] Extratropical cyclone

A fictitious synoptic chart of an extratropical cyclone affecting the UK and Ireland. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the centre of the "low". Note the occluded, cold and warm frontal boundaries.

An extratropical cyclone is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[3]

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,^[3][4] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with anti-cyclones, 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 dewpoint 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.^[5] 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.^[6]

[edit] Surface high pressure systems

High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. 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.^[7][8] High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English,^[9] or A in Spanish,^[10] 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.^[11]

[edit] See also

• Mesoscale meteorology
• Microscale meteorology
• Storm-scale
• Misoscale meteorology
• Outline of meteorology

[edit] References

[edit] External links

• Definition of synoptic scale
• Definition of synoptic meteorology
• Definition of synoptic hours