Weather map

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

A weather map, also known as synoptic weather chart, displays various meteorological features across a particular area at a particular point in time and has various symbols which all have specific meanings. [1] Such maps have been in use since the mid-19th century and are used for research and weather forecasting purposes. Maps using isotherms show temperature gradients, [2] which can help locate weather fronts. Isotach maps, analyzing lines of equal wind speed, [3] on a constant pressure surface of 300 or 250  hPa show where the jet stream is located. Use of constant pressure charts at the 700 and 500 hPa level can indicate tropical cyclone motion. Two-dimensional streamlines based on wind speeds at various levels show areas of convergence and divergence in the wind field, which are helpful in determining the location of features within the wind pattern. A popular type of surface weather map is the surface weather analysis, which plots isobars to depict areas of high pressure and low pressure. Cloud codes are translated into symbols and plotted on these maps along with other meteorological data that are included in synoptic reports sent by professionally trained observers.

Contents

History

Sir Francis Galton, the inventor of the weather map Francis Galton 1850s.jpg
Sir Francis Galton, the inventor of the weather map

The use of weather charts in a modern sense began in the middle portion of the 19th century in order to devise a theory on storm systems. [4] During the Crimean War a storm devastated the French fleet at Balaklava, and the French scientist Urbain Le Verrier was able to show that if a chronological map of the storm had been issued, the path it would take could have been predicted and avoided by the fleet.

In England, the scientist Francis Galton heard of this work, as well as the pioneering weather forecasts of Robert Fitzroy. After gathering information from weather stations across the country for the month of October 1861, he plotted the data on a map using his own system of symbols, thereby creating the world's first weather map. He used his map to prove that air circulated clockwise around areas of high pressure; he coined the term 'anticyclone' to describe the phenomenon. He was also instrumental in publishing the first weather map in a newspaper, for which he modified the pantograph (an instrument for copying drawings) to inscribe the map onto printing blocks. The Times began printing weather maps using these methods with data from the Meteorological Office. [5]

US weather map from 1843 Wea05013 (9627284429) (cropped).jpg
US weather map from 1843

The introduction of country-wide weather maps required the existence of national telegraph networks so that data from across the country could be gathered in real time and remain relevant for all analysis. The first such use of the telegraph for gathering data on the weather was the Manchester Examiner newspaper in 1847: [6]

...led us to inquire if the electric telegraph was yet extended far enough from Manchester to obtain information from the eastern counties...inquiries were made at the following places; and hypothesis were returned, which we append...

It was also important for time to be standardized across time zones so that the information on the map should accurately represent the weather at a given time. A standardized time system was first used to coordinate the British railway network in 1847, with the inauguration of Greenwich Mean Time.

In the US, 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. [7] The U.S. Army Signal Corps inherited this network between 1870 and 1874 by an act of Congress, and expanded it to the west coast soon afterwards. At first, not all the data on the map was used due to a lack of time standardization. The United States fully adopted time zones in 1905, when Detroit finally established standard time. [8] [9]

20th century

Light tables were important to the construction of surface weather analyses into the 1990s US Navy 030209-N-2972R-079 Illustrator Draftsman designs a training slide with the assistance of a light table.jpg
Light tables were important to the construction of surface weather analyses into the 1990s

The use of frontal zones on weather maps began in the 1910s in Norway. Polar front theory is attributed to Jacob Bjerknes, derived from a coastal network of observation sites in Norway during World War I. This theory proposed that the main inflow into a cyclone was concentrated along two lines of convergence, one ahead of the low and another trailing behind the low. The convergence line ahead of the low became known as either the steering line or the warm front. The trailing convergence zone was referred to as the squall line or cold front. Areas of clouds and rainfall appeared to be focused along these convergence zones. The concept of frontal zones led to the concept of air masses. The nature of the three-dimensional structure of the cyclone would wait for the development of the upper air network during the 1940s. [10] Since the leading edge of air mass changes bore resemblance to the military fronts of World War I, the term "front" came into use to represent these lines. [11] The United States began to formally analyze fronts on surface analyses in late 1942, when the WBAN Analysis Center opened in downtown Washington, D.C. [12]

In addition to surface weather maps, weather agencies began to generate constant pressure charts. In 1948, the United States began the Daily Weather Map series, which at first analyzed the 700 hPa level, which is around 3,000 metres (9,800 ft) above sea level. [13] By May 14, 1954, the 500 hPa surface was being analyzed, which is about 5,520 metres (18,110 ft) above sea level. [14] The effort to automate map plotting began in the United States in 1969, [15] with the process complete in the 1970s. A similar initiative was started in India by Indian Meteorological Department in 1969. [16] Hong Kong completed their process of automated surface plotting by 1987. [17]

By 1999, computer systems and software had finally become sophisticated enough to allow for the ability to underlay on the same workstation 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. In the United States, this development was achieved when Intergraph workstations were replaced by n-AWIPS workstations. [18] 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. [19] 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. [20]

Plotting of data

Lowcloudsymbols.gif
Low étage (Sc,St) and upward-growing vertical (Cu, Cb)
Midcloudcymbols.gif
Middle étage (Ac,As) and downward-growing vertical (Ns)
Highcloudsymbols.gif
High étage (Ci,Cc,Cs)
Present weather symbols used on weather maps Weather symbolsNEW2.png
Present weather symbols used on weather maps
Wind barb interpretation Wind barbs.gif
Wind barb interpretation

A station model is a symbolic illustration showing the weather occurring at a given reporting station. Meteorologists created the station model to plot a number of weather elements in a small space on weather maps. Maps filled with dense station-model plots can be difficult to read, but they allow meteorologists, pilots, and mariners to see important weather patterns. A computer draws a station model for each observation location. The station model is primarily used on surface-weather maps, but can also be used to show the weather aloft. A completed station-model map allows users to analyze patterns in air pressure, temperature, wind, cloud cover, and precipitation. [21]

Station model plots use an internationally accepted coding convention that has changed little since August 1, 1941. Elements in the plot show the key weather elements, including temperature, dewpoint, wind, cloud cover, air pressure, pressure tendency, and precipitation. [22] [23] Winds have a standard notation when plotted on weather maps. More than a century ago, winds were plotted as arrows, with feathers on just one side depicting five knots of wind, while feathers on both sides depicted 10 knots (19 km/h) of wind. The notation changed to that of half of an arrow, with half of a wind barb indicating five knots, a full barb ten knots, and a pennant flag fifty knots.

Because of the structure of the SYNOP code, a maximum of three cloud symbols can be plotted for each reporting station that appears on the weather map. All cloud types are coded and transmitted by trained observers then plotted on maps as low, middle, or high-étage using special symbols for each major cloud type. Any cloud type with significant vertical extent that can occupy more than one étage is coded as low (cumulus and cumulonimbus) or middle (nimbostratus) depending on the altitude level or étage where it normally initially forms aside from any vertical growth that takes place. [24] [25] The symbol used on the map for each of these étages at a particular observation time is for the genus, species, variety, mutation, or cloud motion that is considered most important according to criteria set out by the World Meteorological Organization (WMO). If these elements for any étage at the time of observation are deemed to be of equal importance, then the type which is predominant in amount is coded by the observer and plotted on the weather map using the appropriate symbol. Special weather maps in aviation show areas of icing and turbulence. [26]

Types

Alaskan aviation weather map Ifr.gif
Alaskan aviation weather map

Aviation maps

Aviation interests have their own set of weather maps. One type of map shows where VFR (visual flight rules) are in effect and where IFR (instrument flight rules) are in effect. Weather depiction plots show ceiling height (level where at least half the sky is covered with clouds) in hundreds of feet, present weather, and cloud cover. [27] Icing maps depict areas where icing can be a hazard for flying. Aviation-related maps also show areas of turbulence. [28]

Constant pressure charts

An upper level jet streak. DIV areas are regions of divergence aloft, which usually leads to surface convergence and cyclogenesis Jetstreak.png
An upper level jet streak. DIV areas are regions of divergence aloft, which usually leads to surface convergence and cyclogenesis

Constant pressure charts normally contain plotted values of temperature, humidity, wind, and the vertical height above sea level of the pressure surface. [29] They have a variety of uses. In the mountainous terrain of the western United States and Mexican Plateau, the 850 hPa pressure surface can be a more realistic depiction of the weather pattern than a standard surface analysis. Using the 850 and 700 hPa pressure surfaces, one can determine when and where warm advection (coincident with upward vertical motion) and cold advection (coincident with downward vertical motion) is occurring within the lower portions of the troposphere. Areas with small dewpoint depressions and are below freezing indicate the presence of icing conditions for aircraft. [30] The 500 hPa pressure surface can be used as a rough guide for the motion of many tropical cyclones. Shallower tropical cyclones, which have experienced vertical wind shear, tend to be steered by winds at the 700 hPa level. [31]

Use of the 300 and 200 hPa constant pressure charts can indicate the strength of systems in the lower troposphere, as stronger systems near the Earth's surface are reflected as stronger features at these levels of the atmosphere. Isotachs are drawn at these levels, which a lines of equal wind speed. They are helpful in finding maxima and minima in the wind pattern. Minima in the wind pattern aloft are favorable for tropical cyclogenesis. Maxima in the wind pattern at various levels of the atmosphere show locations of jet streams. Areas colder than −40 °C (−40 °F) indicate a lack of significant icing, as long as there is no active thunderstorm activity. [30]

Surface weather analysis

Streamline analysis of the tropical Pacific Ocean Streamlineanalysispacific.gif
Streamline analysis of the tropical Pacific Ocean

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. Isotherms can be drawn on these maps, which are lines of equal temperature. Isotherms are drawn normally as solid lines at a preferred temperature interval. [2] They show temperature gradients, which can be useful in finding fronts, which are on the warm side of large temperature gradients. By plotting the freezing line, isotherms can be useful in determination of precipitation type. Mesoscale boundaries such as tropical cyclones, outflow boundaries and squall lines also are analyzed on surface weather analyses.

Isobaric analysis is performed on these maps, which involves the construction of lines of equal mean sea level pressure. The innermost closed lines indicate the positions of relative maxima and minima in the pressure field. The minima are called low-pressure areas while the maxima are called high-pressure areas. Highs are often shown as H's whereas lows are shown as L's. Elongated areas of low pressure, or troughs, are sometimes plotted as thick, brown dashed lines down the trough axis. [32] Isobars are commonly used to place surface boundaries from the horse latitudes poleward, while streamline analyses are used in the tropics. [33] A streamline analysis is a series of arrows oriented parallel to wind, showing wind motion within a certain geographic area. "C"s depict cyclonic flow or likely areas of low pressure, while "A"s depict anticyclonic flow or likely positions of high-pressure areas. [34] An area of confluent streamlines shows the location of shearlines within the tropics and subtropics. [19]

See also

Related Research Articles

<span class="mw-page-title-main">Cyclone</span> Large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

<span class="mw-page-title-main">Surface weather analysis</span> Type of weather map

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

<span class="mw-page-title-main">Freezing rain</span> Rain maintained at temperatures below freezing

Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, and then freeze upon impact with any surface they encounter, including the ground, trees, electrical wires, aircraft, and automobiles. The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces. The METAR code for freezing rain is FZRA.

<span class="mw-page-title-main">Anticyclone</span> Weather phenomenon of high pressure, as opposed to a cyclone

An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.

<span class="mw-page-title-main">High-pressure area</span> Region with higher atmospheric pressure

A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interplays between the relatively larger-scale dynamics of an entire planet's atmospheric circulation.

<span class="mw-page-title-main">Low-pressure area</span> Area with air pressures lower than adjacent areas

In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:

<span class="mw-page-title-main">Synoptic scale meteorology</span> 1000-km-order method of measuring weather systems

In meteorology, the synoptic scale is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high- and low-pressure areas seen on weather maps are synoptic-scale systems, driven by the location of Rossby 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, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together".

<span class="mw-page-title-main">Pressure system</span> Relative peak or lull in the sea level pressure distribution

A pressure system is a peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured 87 kilopascals (26 inHg) and the highest recorded 108.57 kilopascals (32.06 inHg). High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiationized cooling an area receives. Pressure systems cause weather to be experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps.

<span class="mw-page-title-main">Weather front</span> Boundary separating two masses of air of different densities

A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.

<span class="mw-page-title-main">Mesoscale convective system</span> Complex of thunderstorms organized on a larger scale

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

<span class="mw-page-title-main">Trough (meteorology)</span> Elongated region of low atmospheric pressure

A trough is an elongated region of relatively low atmospheric pressure without a closed isobaric contour that would define it as a low pressure area. Since low pressure implies a low height on a pressure surface, troughs and ridges refer to features in an identical sense as those on a topographic map.

<span class="mw-page-title-main">Eye (cyclone)</span> Central area of calm weather in a tropical cyclone

The eye is a region of mostly calm weather at the center of a tropical cyclone. The eye of a storm is a roughly circular area, typically 30–65 kilometers in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds of the cyclone occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.

<span class="mw-page-title-main">Dvorak technique</span> Subjective technique to estimate tropical cyclone intensity

The Dvorak technique is a widely used system to estimate tropical cyclone intensity based solely on visible and infrared satellite images. Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The primary patterns used are curved band pattern (T1.0-T4.5), shear pattern (T1.5–T3.5), central dense overcast (CDO) pattern (T2.5–T5.0), central cold cover (CCC) pattern, banding eye pattern (T4.0–T4.5), and eye pattern (T4.5–T8.0).

<span class="mw-page-title-main">Extratropical cyclone</span> Type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to severe gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

The history of surface weather analysis concerns the timetable of developments related to surface weather analysis. Initially a tool of study for the behavior of storms, surface weather analyses became a work in progress to explain current weather and as an aid for short term weather forecasting. Initial efforts to create surface weather analyses began in the mid-19th century by using surface weather observations to analyze isobars, isotherms, and display temperature and cloud cover. By the mid-20th century, much more information was being placed upon the station models plotted on weather maps and surface fronts, per the Norwegian cyclone model, were being analyzed worldwide. Eventually, observation plotting went from a manual exercise to an automated task for computers and plotters. Surface analysis remains a manual and partially subjective exercise, whether it be via hand and paper, or via a workstation.

<span class="mw-page-title-main">Station model</span> Type of meteorological illustration

In meteorology, station models are symbolic illustrations showing the weather occurring at a given reporting station. Meteorologists created the station model to fit a number of weather elements into a small space on weather maps. This allows map users to analyze patterns in atmospheric pressure, temperature, wind speed and direction, cloud cover, precipitation, and other parameters. The most common station plots depict surface weather observations although upper air plots at various mandatory levels are also frequently depicted.

<span class="mw-page-title-main">Outline of meteorology</span> Overview of and topical guide to meteorology

The following outline is provided as an overview of and topical guide to the field of Meteorology.

<span class="mw-page-title-main">Surface weather observation</span> Fundamental data used for weather forecasts

Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using weather observers to augment the otherwise automated weather station. The ICAO defines the International Standard Atmosphere (ISA), which is the model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earth's atmosphere, and is used to reduce a station pressure to sea level pressure. Airport observations can be transmitted worldwide through the use of the METAR observing code. Personal weather stations taking automated observations can transmit their data to the United States mesonet through the Citizen Weather Observer Program (CWOP), the UK Met Office through their Weather Observations Website (WOW), or internationally through the Weather Underground Internet site. A thirty-year average of a location's weather observations is traditionally used to determine the station's climate. In the US a network of Cooperative Observers make a daily record of summary weather and sometimes water level information.

<span class="mw-page-title-main">Cold front</span> Leading edge of a cooler mass of air

A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone, at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak, a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.

<span class="mw-page-title-main">Cold-core low</span> Cyclone with an associated cold pool of air at high altitude

A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day.

References

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