Surface weather analysis

NWSweatherfronts.gif|thumb|right|300px|A guide to the symbols for weather fronts that may be found on a weather map:
1. cold front
2. warm front
3. stationary front
4. occluded front
5. surface trough
6. squall line
7. dry line
8. tropical wave
]]

"Cold front" redirects here. For the first season episode of Enterprise, see Cold Front (Enterprise episode).

A weather map, or surface analysis, provides a view of weather elements over a specified geographical area at a specified time. Weather map pioneers include William Redfield http://www.magma.ca/~jdreid/redfield.htm, William Reid http://www.magma.ca/~jdreid/reid.htm, Elias Loomishttp://www.magma.ca/~jdreid/lstorms.htm, and Sir Francis Galton who created the first weather maps in order to devise a theory on storm systems. Weather maps are created by plotting or tracing the values of relevant quantities such as pressure, temperature, cloud cover, and others, onto a geographical map. The data can be either measurements or forecast values. Weather maps will often have symbols on them to show frontal systems, cloud cover, precipitation, or other important information. For example, an H may represent high pressure, assuring good and fair weather. An L, on the other hand, may represent low pressure, so be prepared for precipitation. Weather maps used in aviation often contain symbols for turbulence and icing.

History of Surface Analysis

The use of weather charts in a modern sense began in the nineteenth century as the 1845 telegraph made it possible to gather weather information from multiple distant locations quickly enough to preserve its value for real-time applications. The Smithsonian Institution developed its network of observers over much of the central and eastern United States between the 1840s and 1860s once Joseph Henry took the helm. http://www.si.edu/archives/ihd/jhp/joseph03.htm Beginning in 1849, the Smithsonian started producing surface analyses on a daily basis using the 150 stations in their network. http://www.weather.gov/pa/history/timeline.php The United States Army Signal Corps, which evolved into the modern National Weather Service, inherited this network between 1870 and 1874 by an act of Congress, and expanded it to the west coast soon afterwards. Each day at 7:30 AM local time, all stations would telegraph in their observations to the central office which would then plot the information on a map upon which isobars, or lines of equal pressure, would be drawn which would identify centers of high and low pressure, as well as squall lines. All the data on the map was not taken at exactly the same time in the early days of these analyses because of a lack of time standardization. The first attempts at time standardization may have taken hold in the Great Britain by 1855, but in the United States standard time did not come to pass until 1883, when time zones started to come into use across America for railroad use. The entire United States did not finally come under the influence of time zones until 1900, when Detroit finally fell into line.

Other countries started preparing surface analyses in the nineteenth century as well. In Australia, the first weather map showed up in print media in 1877. http://www.bom.gov.au/lam/climate/levelthree/analclim/earlym.htm Japan's Tokyo Meteorological Observatory, the forerunner of the JMA, began constructing surface weather maps in 1883. http://www.jma.go.jp/jma/en/History/indexe_his.htm

The use of frontal zones on weather maps did not appear until the introduction of the Norwegian cyclone model in the late 1910s, despite Loomis' earlier attempt at a similar notion in 1841. http://www.magma.ca/~jdreid/ Despite the introduction of the Norwegian cyclone model just after World War I, the United States did not formally analyze fronts on surface analyses for an additional generation, until after World War II. Manually plotted maps became automated in the 1970s, and by the late 1990s, computer systems had finally become sophisticated enough to allow for the ability to underlay satellite imagery, radar imagery, and model-derived fields such as atmospheric thickness and frontogenesis in combination with surface observations to make for the best possible surface analysis. By 2001, the various surface analyses done within the National Weather Service were combined into the Unified Surface Analysis, which is issued every six hours and combines the analyses of four different centers. http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf

Recent advances in both the fields of meteorology and Geographic Information Systems have made it possible to devise finely tailored products that take us from the traditional weather map into an entirely new realm. Weather information can quickly be matched to relevant geographical detail. For instance, icing conditions can be mapped onto the road network. This will likely continue to lead to changes in the way surface analyses are created and displayed over the next several years.

Fronts

Fronts in meteorology are the leading edges of air masses with different density (e.g., air temperature or humidity). When a front passes over an area, it is marked by changes in temperature, moisture, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern. Cold fronts are often closely associated with low pressure systems, normally lying at the leading edge of high pressure systems and, in the case of the polar front, at approximately the equatorward edge of the high-level polar jet. Fronts are generally guided by winds aloft, though normally at lesser speeds, and travel from west to east. This movement is mainly due to the Coriolis effect, caused by the earth spinning about its axis. Frontal zones can be contorted by geographic features like mountains and large bodies of water.

Types of frontal zones

Cold Front

A cold front is defined as the leading edge of a mass of air which is colder than the air in front of it. http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/cfrnt/def.rxml. The colder air, being denser, wedges under the less dense warmer air, lifting it, causing the formation of mostly cumuliform (puffy, cotton-ball-like) clouds. The passage of a cold front usually results in velocity changes in winds and creates vertical movement of air (turbulence) and can set off atmospheric disturbances such as rainshowers, thunderstorms, squall lines, tornadoes, and snowstorms ahead of and immediately behind the moving cold front. The air behind the cold front is generally drier and cooler than that which it is replacing. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel.

Warm Front

A warm front is defined as the leading edge of a mass of warm air. Warm fronts move more slowly than cold fronts and consist of generally stable air, i.e., little vertical air movement, causing the formation of mostly stratiform clouds. Warm fronts usually bring steadier, lighter precipitation in the form of rain, fog or snow which can last from a few hours to several days. On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of travel.

Occluded Fronts/Trowals

Occluded fronts are formed when a cold front overtakes a warm front, forcing the warm air aloft. http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/ofdef.rxml The two fronts curve up naturally into the point of occlusion, also known as a triple point. http://www.srh.noaa.gov/oun/severewx/glossary4.php#t 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 airmass. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. Occluded fronts usually form around mature low pressure areas. They are special forms of a Trowal (Trough of Warm air Aloft), but not all trowals are surface-based occlusions. Many times, trowals end up being analyzed as surface troughs that appear inside a dense, cool airmass. http://amsglossary.allenpress.com/glossary/browse?s=t&p=52

Stationary Front

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. http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/sfdef.rxml A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change. Stationary fronts are more numerous in the summer months. Stationary fronts are marked on weather maps with alternating red bumps and blue spikes pointing in opposite directions, indicating no significant movement. Prolonged precipitation associated with stationary fronts can lead to flooding snowfalls during the winter, summer, and autumn months.

Mesoscale features

Shear Line

http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line A shear line is an area in a low pressure trough, usually in the tropics, within which wind direction changes significantly over a relatively short distance. The area is marked by an increase in cumuluform clouds, often including towering cumulus, and rainshowers. It may become more active with thunderstorms, and the turbulence and circular motion of winds may assist in the formation of a tropical storm. A shear line is depicted as a line of red dots and dashes.

Dry Line

A similar phenomenon to a frontal zone is the dry line, which is the boundary between moist and dry air. By definition, drylines form in response to downsloped air from higher terrain mixed to the surface during daytime heating, and in three dimensions the dry line resembles a cold front, though it normally sloshes eastward during the day, and westward at night. http://ams.allenpress.com/pdfserv/10.1175%2F1520-0469(1992)049%3C1606:FADVOT%3E2.0.CO%3B2 This is caused by density differences enhanced by the temperature of the airmass on both sides switching character during the day and night, which is a factor of the moisture in the air. The drier an airmass is, the easier it is to heat or cool it. When a dry line passes eastwards past an area, there is an associated decrease in humidity. The dryline retreat to the west is associated with an increase in moisture. The boundary causes a great deal of surface-based moisture convergence in the Great Plains, and tends to lead to powerful storms since its presence is also coincident with strong winds aloft. A dryline is depicted on NWS surface analyses as a brown line with scallops facing into the moist sector. Drylines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion. http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/dfdef.rxml

Outflow Boundaries/Squall Lines

Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but they can outrun cold fronts in a pattern where the upper level jet splits 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. When the convection is strong and linear/curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. http://www.ofcm.gov/slso/pdf/slsochp2.pdf Even weaker and less organized areas of thunderstorms will 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. http://www.geographic.org/climate/o.html These features will commonly be depicted in the warm season across the United States on surface analyses, and they lie within surface troughs. If outflow boundaries or squall lines form over arid regions, a haboob may result. http://www.wrcc.dri.edu/ams/glossary.html#H Squall lines are depited on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN, while outflow boundaries are depicted as troughs with a label of OUTFLOW BNDRY.

Lee Trough

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. If moisture pools along with boundary during the warm season, it can be the focus of diurnal thunderstorms. http://amsglossary.allenpress.com/glossary/search?id=lee-trough1

Sea/Lake/River/Land Breeze Fronts

Sea/lake/river breeze fronts occur mainly on sunny days when the landmass warms up above the water temperature. Since the specific heat of water is so significant compared to most other substances, there is little diurnal change in ocean/lakes/bays even on the sunniest days...usually limited to 1-2F or 1C. During the afternoon, sea breezes move inland when relatively cooler/milder air from the water body moves inland to fill in the gap left by lowered pressures caused by the relatively warm air over the landmass. This process reverses at night, leading to a land breeze and wind acceleration offshore. If enough moisture exists, thunderstorms can form along sea/lake/river/land breeze fronts which then can send out their outflow boundaries, which can lead to chaotic wind/pressure regimes if winds are light and variable with height. Like all other surface features, sea/lake/river/land breeze fronts also lie inside troughs, but if surface data is not dense enough, this trough may not be readily apparent. http://amsglossary.allenpress.com/glossary/search?p=1&query=sea+breeze

Types of precipitation produced by fronts

Fronts are the principal cause of significant weather. 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 under a mass of warmer air. If the temperature differences of the two air masses involved are large and the turbulence is extreme, "roll clouds" and thus tornadoes, may occur. http://amsglossary.allenpress.com/glossary/search?p=1&query=convection In the warm season, lee troughs, sea/lake/river/land breezes, outflow boundaries, and trowals/occlusions can lead to convection if enough moisture is available. Orographic precipitation refers to precipitation generated 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 also sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain. Precipitation along warm fronts, however, is relatively steady, as in rain or drizzle. Fog, sometimes extensive and dense, is also often present in pre-warm-frontal areas. drizzlehttp://amsglossary.allenpress.com/glossary/search?id=orographic-lifting1

References

External links

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