Surface weather analysis

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

"Cold front" redirects here. For the first season episode of Star Trek: 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 ^[1], William Reid ^[2], Elias Loomis^[3], 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 (in English) or A (in Spanish) may represent high pressure, assuring good and fair weather. An L (in English) or B (in Spanish), 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.

[edit] History of surface analysis

The use of weather charts in a modern sense began in the mid-nineteenth century. The invention of the telegraph in 1845 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. ^[4] Beginning in 1849, the Smithsonian started producing surface analyses on a daily basis using the 150 stations in their network. ^[5] 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. ^[6] Japan's Tokyo Meteorological Observatory, the forerunner of the Japan Meteorological Agency, began constructing surface weather maps in 1883. ^[7]

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.^[8] The use of the term "front" to describe a weather line on a map or the interface between air masses that the line symbolizes came from the line's resemblance to the military fronts of World War I^[9]).

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. ^[10]

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.

[edit] Fronts

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/shear line
7. dry line
8. tropical wave

Fronts in meteorology are the leading edges of air masses with different density (e.g., air temperature and/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, but they normally move at lesser speeds. In the northern hemisphere, they usually travel from some west to east direction (even though they can move in a more north-south direction as well). Movement is largely due to the pressure gradient force (due to horizontal differences in atmospheric pressure) and 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.

[edit] Types of frontal zones

[edit] Cold front

A cold front is defined as the leading edge of a mass of air which is usually colder and drier than the air mass in front of it, outside of terrain effects. ^[11]. 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. Cold fronts can move up to twice as fast as warm fronts.

[edit] Warm front

A warm front is defined as the leading edge of a mass of warm air. Warm fronts move more slowly than the cold front which usually follows. If the warm air mass is stable, clouds ahead of the warm front are mostly stratiform and rainfall gradually increases as the front approaches. At the front itself, the clouds can reach the surface as fog. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage, thundershowers may continue. These may become organized ahead of the following cold front as a squall line. 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 cyclone frontal depiction

[edit] Occluded front

An occluded front is formed during the process of cyclogenesis when a cold front overtakes a warm front.^[12] The two fronts curve up naturally into the point of occlusion, also known as a triple point. ^[13]

There are two types of occlusion, the warm and the cold. In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air.

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.

[edit] 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. ^[14] 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 half-circles and blue spikes pointing in opposite directions, indicating no significant movement.

[edit] Mesoscale features

Mesoscale features are smaller than synoptic scale systems like fronts, but larger than storm-scale systems like thunderstorms. Horizontal dimensions generally range from around 50 miles to several hundred miles.

[edit] Shear line

A shear line is an area where wind direction changes significantly over a relatively short distance.^[15] A typical example is found along a low pressure trough, especially in the tropics. Wind shear with a net inflow at the surface is also a convergence zone, where air is lifted and typically marked by an increase in cumuluform clouds, often including towering cumulus, and rainshowers. If the shear line becomes active with thunderstorms, it may support formation of a tropical storm. A shear line is depicted as a line of red dots and dashes.

[edit] Dry line

A similar phenomenon to a frontal zone is the dry line, which is the boundary between airmasses with significant moisture differences. Near the surface during daylight hours, warm moist air is more dense than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the cooler and drier air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.^[16]

The dryline may occur anywhere on the globe in regions intermediate between desert areas and warm seas. The southern plains west of the Mississippi in the U.S. are a particularly favored location. The dryline normally sloshes eastward during the day, and westward at night. 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. ^[17]

[edit] Outflow boundaries/squall lines

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

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 or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise. ^[18] 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. ^[19] 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. ^[20] Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or SQUALL LINE, while outflow boundaries are depicted as troughs with a label of OUTFLOW BNDRY.

[edit] 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. ^[21]

Idealized circulation pattern associated with a sea breeze

[edit] 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. ^[22]

[edit] 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, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme due to wind shear and the presence of a jet max, "roll clouds" and tornadoes may occur. ^[23] In the cold 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 often present in pre-warm-frontal areas. ^[24]

It should be noted that not all fronts produce precipitation or even clouds. Moisture must be present in the air mass which is being lifted.

There are also differences between fronts over continental versus ocean areas. Cold fronts on land tend to clear out quickly upon passage while cold fronts over the ocean can be as rainy behind the front as ahead of it.

Plotted Weather Symbols Seen on Weather Maps

[edit] Plotted Symbols on Weather Maps

In order to analyze a weather map, a station model is plotted at each point of observation. In the station model, the temperature, dewpoint, wind, sea level pressure, pressure tendency, and ongoing weather are plotted. Once a map has a field of station models plotted, the analyzing of isobars (lines of equal pressure), isollobars (lines of equal pressure change), isotherms (lines of equal temperature), and isotachs (lines of equal wind speed) can be easily contoured. Symbols for the various types of weather are not straightforward. To the right is a list of weather symbols concerning precipitation commonly seen on surface analyses.

[edit] See Also

Wikimedia Commons has media related to: Atmospheric fronts

• Extratropical cyclone
• Cyclogenesis
• Wind Shear
• Dry line
• Orographic precipitation

[edit] References

[edit] External links

• "The Mid-Latitude Cyclone"
• Norwegian Cyclone Model -NWS
• Unified Surface Analysis Manual - NWS
• Unified Surface Analysis - NWS
• Glossary of Meteorology
• Cold Front Page