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SYNOP
SYNOP
SYNOP
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SYNOP (surface synoptic observations) is a numerical code (called FM-12 by WMO) used for reporting weather observations made by staffed and automated weather stations. SYNOP reports are typically sent every six hours by Deutscher Wetterdienst on shortwave and low frequency using RTTY. A report consists of groups of numbers (and slashes where data is not available) describing general weather information, such as the temperature, barometric pressure and visibility at a weather station. It can be decoded by open-source software such as seaTTY, metaf2xml or Fldigi.

SYNOP information is collected by more than 7600 staffed and unstaffed meteorological stations and more than 2500 mobile stations around the world and is used for weather forecasting and climatic statistics. The format of the original messages is abbreviated, and some items are coded.[1]

Message format

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Following is the general structure of a SYNOP message. The message consists of a sequence of numeric groups, which may also contain slashes (indicating missing data) in addition to numeric digits. Leading numbers are fixed group indicators that indicate the type of observation following, and letters are replaced with numbers giving the weather data.[2][3] Messages from shipboard weather stations, and in different regions of the world, use variations on this scheme. 

 YYGGiw IIiii iRiXhVV Nddff (00fff) 1snTTT 2snTdTdTd 3PoPoPoPo 4PPPP 5appp 6RRRtR 7wwW1W2 8NhCLCMCH (9GGgg)
  • YYGGiw: the date and time of the observation; YY for the day of the month, GG for the hour of the observation in UTC; iw for the manner of wind observation (a code number: 0 for estimated wind speed in meters per second, 1 for measured wind speed in meters per second, 3 and 4 likewise but in knots, or slash for no wind speed observations).
  • IIiii: weather station identification code; II for a block number allocated (by the WMO) to a country or a region of the world, for example 02 for Scandinavia or 72 and 74 for the continental US; iii is the code of an individual station within a block. (For example, 02993 is the code of the weather station on Märket, 74794 of Cape Canaveral).[4]
  • iRiXhVV:
    • iR indicates whether precipitation data is included or omitted. This is a code number from 0 to 4, with 0, 1 and 2 meaning data is included, and 3 and 4 indicating no precipitation data.
    • iX is a code number indicating the manner of station operation, and the format used in group 7wwWW; codes 1, 2 and 3 indicate a staffed station, while codes 4 to 7 indicate an automatic station.
    • h indicates the height above the surface for the base of the lowest cloud seen: 0 means from 0 to 100 feet or 0 to 50 meters, 9 means the base of clouds is 2500 meters or higher or that there are no clouds.
    • VV indicates horizontal surface visibility:
      • For codes 00 to 50, this indicates visibility in tenths of a kilometer (hectometers), for example "15" means 1.5 km.
      • For codes 56 to 80, 50 is subtracted, and the resulting number indicates visibility in kilometers, for example "66" means 16 km.
      • Codes 81 to 88 indicate visibility in a multiple of 5 km; "81" for 35 km, "88" for 70 km. Code 89 indicates visibility greater than 70 km;
      • Codes 90 to 99 are used for shipboard observations, from "90" for less than 1⁄16 mile visibility, "95" for 1 mile, "99" for greater than 30 miles.
  • Nddff and 00fff:
    • N: total cloud cover in eighths of the sky (oktas); "0" for no clouds, "4" for half (4⁄8) of the sky obscured, "8" for total cloud cover, "9" for an obscured sky or a situation where cloud cover can't be estimated, "/" for no measurement in the case of automatic stations.
    • dd for true wind direction in tens of degrees, with "00" meaning "no wind", "18" for wind from the south (175° to 184°), "36" for wind from the north (355° to 004°).
    • ff for wind speed, in the units specified in the YYGGiw group. If wind speed is 99 units or more, this group will have the code "99" and will immediately be followed by the group 00fff, with the wind speed indicated there instead.
  • 1snTTT: air temperature. Code sn indicates the sign, 0 for positive, 1 for negative degrees; TTT has the temperature in tenths of a degree Celsius.
  • 2snTdTdTd: dew point temperature. Like the preceding 1snTTT group, sn stands for the sign, and TdTdTd has the temperature in tenths of a degree Celsius. If sn is 9, the last three digits are instead relative humidity in percent, from "000" to "100".
  • 3PoPoPoPo: air pressure at station level, in tenths of a hectopascal. If the pressure is more than 999.9 hPa, the leading thousands digit is dropped; for example 30240 means a pressure of 1024.0 hPa.
  • 4PPPP: air pressure at sea level, in tenths of a hectopascal, derived from station pressure.
  • 5appp: three-hour pressure tendency. The digit a encodes the manner of pressure change, for example "3" means "decreasing or steady, then increasing; or increasing, then increasing more rapidly". Codes 1 to 3 indicate higher pressure than 3 hours ago, codes 6 to 8 indicate lower pressure, while codes 0, 4 and 5 indicate approximately the same pressure. Digits ppp indicate the actual pressure change, in tenths of a hectopascal.
  • 6RRRtR: amount of precipitation, in millimeters. Digit tR indicates the length of time covered by this group, such as a measurement over the past 6, 12, 18 or 24 hours.
  • 7wwW1W2: present (ww) and past (W1W2) weather. Staffed and automatic stations use different formats and tables for this group. These are looked up from a table, with various different codes, such as "35" for "severe duststorm or sandstorm, has begun or has increased during the preceding hour".
  • 8NhCLCMCH: cloud types. Nh indicates amount of low-altitude clouds (in oktas, like in the Nddff group), or if none, the amount of medium-altitude clouds. CL, CM and CH indicate the types of low, medium, and high-altitude clouds present, with codes looked up from tables.
  • 9GGgg: actual time of observation, in hours and minutes UTC, used when the actual time differs more than 10 minutes from the time reported in the YYGGiw group.

After this first section, stations may include additional sections, prefixed by 222// (section 2, for staffed coastal stations, reporting sea surface temperature and wave data), 333 (section 3, used only in some areas of the world, for the "state of the sky in the tropics"), or 555 (various national code groups).

Example message

[edit]

This observation was from April 1, 2022, from LaGuardia Airport in New York City.[5] 

 01124 72503 12566 63015 10106 20050 30003 40016 53048 60071 91151
 333 10178 20106 70079 91021
  • 01124: First day of the month (01), 12:00 UTC (12), with wind speed in knots, measured by anemometer (4).
  • 72503: The station's WMO index, New York, La Guardia Airport.
  • 12566: Precipitation data is included in section 1 (1), station is staffed (2), past weather observations (group 7wwW1W2) not included (2), base of lowest observed cloud is from 600 to 999 m (5), horizontal visibility at surface 16 km (66).
  • 63015: Total cloud cover 6⁄8 of the sky (6), wind direction is between 295° and 304° (30), wind speed 15 knots (15).
  • 10106: Temperature 10.6 °C.
  • 20050: Dew point temperature 5.0 °C.
  • 30003: Pressure at station level 1000.3 hPa.
  • 40016: Calculated pressure at sea level: 1001.6 hPa.
  • 53048: Over the last three hours, pressure has first decreased and then increased, to end up higher than in the last report (3); pressure change since last report 4.8 hPa (048).
  • 60071: 7.0 mm precipitation (007) over the past six hours (1).
  • 91151: Actual time of observation 11:51 UTC.
  • 333: Start of section 3.
  • 10178: Maximum temperature over the past day 17.8 °C.
  • 20106: Minimum temperature over the past day 10.6 °C.
  • 70079: 7.9 mm precipitation over the past 24 hours.
  • 91021: Special phenomena.

Although this coded data is still available from three American universities it has now been replaced by a universal digital coding system so data can be shared in the same format whatever the source of the observations. This enables Synop, Metar, upperair and satellite data to be processed by a common computer system.

The short wave radio transmission of Synop data was common in the 1980s from Bracknell or Paris but this is now redundant. Synop data is available as downloadable files from a number of internet sites including the College of DuPage.[6]

See also

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References

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Surface and SYNOP datasets

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

SYNOP

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SYNOP (surface synoptic observations) is a numerical code, designated as FM-12 by the (WMO), used for reporting standardized observations from fixed land stations. It encodes key meteorological parameters—including , air temperature, , and speed, , present and past phenomena, amount and height, and —into a compact, alphanumeric format for efficient global transmission and analysis. Developed as part of WMO's international standards, SYNOP facilitates the exchange of surface data essential for synoptic-scale , climate monitoring, and , with reports typically issued at standard observation times of 00, 06, 12, and 18 UTC, supplemented by intermediate hours as needed. The code's structure consists of sequential five-digit groups (with some exceptions), beginning with identification details such as the reporting station's WMO index number, date, and time (e.g., section 0: IIiii YYGGid), followed by groups in section 1 encoding surface observations such as pressure, temperature, visibility, and weather codes. Optional sections cover supplementary information, including trends in recent developments or national-specific data, ensuring flexibility while maintaining universality. Both manned and automated stations generate SYNOP reports, which are disseminated via the WMO's Global Telecommunication System (GTS) for real-time use in numerical weather prediction models and synoptic chart preparation. Despite the rise of binary formats like BUFR, SYNOP remains widely used due to its simplicity and compatibility with legacy systems, supporting ongoing meteorological cooperation under WMO regulations.

Overview

Definition and Purpose

SYNOP, formally designated as the FM-12 code form by the (WMO), is a standardized numerical used for encoding routine surface synoptic observations from fixed land stations, whether manned or automated. It serves as a concise, machine-readable format to report essential meteorological parameters, including and speed, , present and past phenomena, and type, air , dew-point , and reduced to mean . This encoding enables the systematic collection and transmission of surface data critical for synoptic-scale analysis, which involves studying large-scale atmospheric patterns to understand weather systems. The primary purpose of SYNOP is to facilitate the international exchange of observational data within the WMO's World Weather Watch (WWW) Global Observing System, supporting operational , , , and climatological records. By standardizing reports from land-based stations, SYNOP ensures interoperability across global networks, allowing meteorologists to integrate data into (NWP) models for short- to medium-range forecasts. Unlike the FM-13 SHIP code for marine observations or the FM-35 TEMP code for upper-air soundings, SYNOP is specifically tailored to fixed land stations, focusing on surface-level conditions without vertical profiling or sea-specific elements like . Key benefits of SYNOP include its role in promoting real-time data dissemination through traditional alphanumeric text formats or modern binary systems like BUFR (Binary Universal Form for the Representation of meteorological data), which enhance efficiency and reduce transmission errors in global monitoring. This standardization has been instrumental in building a unified for international collaboration, underpinning advancements in prediction accuracy and disaster preparedness.

History and Development

The SYNOP code originated from early 20th-century international weather telegraphy codes developed under the International Meteorological Organization (IMO), the WMO's predecessor founded in , to enable efficient global exchange of surface observations via limited bandwidth systems like telegraph and radio. These codes emphasized brevity for cost-effective transmission, encoding key parameters such as , , and speed, , and present weather using numeric groups and standardized symbols, building on 19th-century foundations like the 1853 Brussels Maritime Conference that first coordinated ship-based reports. The IMO's technical commissions refined these formats in the 1920s and 1930s, including the 1929 Copenhagen Code for wireless reports, which influenced land-based synoptic practices by prioritizing essential data for weather chart analysis across borders. Following the WMO's establishment in , the SYNOP code was formalized in 1957 as part of the International Code for surface observations at land stations, designated as code form FM-12 in WMO Publication No. 306 (Manual on Codes). This standardization, approved through WMO's Executive Council and Congress, unified disparate national and IMO-era formats into a single alphanumeric system for real-time synoptic reporting, supporting the Global Observing System's expansion during the (1957–1958). The 1957 code maintained telegraphic conciseness while ensuring for international data dissemination via the Global Telecommunication System. A key milestone occurred in the 1960s with the widespread adoption of FM-12 as the core format for fixed land stations, enabling routine 6-hourly synoptic observations worldwide and integrating into numerical weather prediction models. In the 1980s, revisions addressed the rise of automated stations by introducing supplementary code tables, such as Table 4680 for automatic weather station reports, to bridge gaps between manual human observations and machine-generated data, including higher-frequency reporting and reduced subjectivity in phenomena like cloud cover. These updates, implemented progressively from 1982, expanded the code's flexibility without altering its core structure. The SYNOP code's evolution has been guided by WMO's Commission for Basic Systems (CBS), responsible for data representation standards, and Regional Associations, which adapt implementations to local needs like varying observation densities. Challenges in transitioning from manual to automated reporting included accommodating additional variables, such as solar and terrestrial radiation data added in post-1980 editions via optional sections (e.g., Section 5), while preserving brevity for legacy systems; this shift supported integration with digital formats like BUFR in the 2010s, where SYNOP data is encoded as binary equivalents for efficient machine processing without fully replacing the alphanumeric form. By the 2010s, CBS-led migrations emphasized hybrid use, ensuring SYNOP's continued role in global networks amid the push toward table-driven codes.

Message Format

Overall Structure

The SYNOP message, formally known as code form FM 12 in the (WMO) standards, follows a standardized high-level organization designed for efficient transmission of surface weather observations from fixed land stations. It begins with a header identifying the reporting station, typically comprising the station's WMO index number (IIiii), followed by groups denoting the time of observation. The core content is then divided into 5 to 8 main sections, each represented by one or more fixed-length groups of five digits (or characters), ensuring a compact and machine-readable format. These sections collectively encode essential meteorological data, with the message ending after the final data group. The structure emphasizes a clear division between mandatory and optional sections to balance completeness with flexibility. Sections I through V are mandatory and cover foundational elements: Section I for identification (station ID and time of observation), Section II for , Section III for , Section IV for , present and past phenomena, and clouds, and Section V for and direction, temperature, and . Sections VI through VIII are optional or supplementary, included only when relevant data are available; these address trends in recent developments or national-specific data (Section VI), additional atmospheric variables (Section VII), and specialized or national extensions (Section VIII). This tiered approach allows stations to report core global data consistently while accommodating regional or local variations without inflating message size. within groups is indicated by /, and non-applicable groups are omitted. SYNOP messages are formatted as plain text strings, typically ranging from 30 to 50 characters in length for standard reports, though this can extend with additional optional groups. They are transmitted at regular intervals—every 3 hours (00, 03, 06, 09, 12, 15, 18, 21 UTC) for main and intermediate synoptic times, or every 6 hours for reduced schedules—via the WMO Global Telecommunication System (GTS) to facilitate real-time international exchange. This structure, standardized since the mid-20th century, supports reliable automated parsing and integration into weather analysis systems worldwide.

Core Sections Breakdown

The core sections of a SYNOP message form the foundational structure for reporting essential surface weather observations, sequenced to facilitate automated parsing and global interoperability. These mandatory groups, labeled I through V, encode key identification, temporal, hydrometeorological, and atmospheric data in a compact, numeric format defined by the (WMO). Each section follows the previous one without delimiters other than the fixed group lengths, ensuring a logical flow from station context to detailed measurements. Section I: Ship or Fixed Station Identification
This initial section identifies the reporting platform using a standardized 5-digit , crucial for locating and validating the source. For fixed land stations, the code comprises a 1- or 2-digit WMO block number followed by a 3- or 4-digit station identifier, such as 12345, which denotes a specific land-based site within a designated geographic region. It also includes the time of observation as YYGGid, where YY is day (01-31), GG hour (00-23) UTC, i wind estimation indicator, and d observation period. Mobile platforms like ships use a distinct prefix, such as 99 followed by or position data, to distinguish them from fixed sites and enable tracking of transient reports. This encoding ensures unambiguous attribution in international data exchanges. Section II: Pressure Data
Section II reports surface values and tendency, providing indicators of atmospheric trends. It includes station-level (3PoPoPoPo in tenths of hPa), mean sea-level (4PPPP in tenths of hPa), and tendency (5appp, where a is the characteristic of change 0-8, and ppp the net change in tenths of hPa over 3 hours). This section supports synoptic analysis by highlighting evolution, a key driver of systems. Section III: Precipitation
Dedicated to hydrometeorological measurements, Section III reports accumulated to quantify recent rainfall or snowfall intensity and duration. It includes totals via group 6RRRtr, where RRR codes the amount in millimeters (000-999, / for trace), t the time interval (e.g., 1 for 6 hours, 6 for 24 hours), and r the type (0 , 1 , etc.). A separate earlier indicator iR notes if occurred since last report. These details are vital for tracking convective activity and flood risks without overwhelming the message length. Section IV: Visibility, Present and Past Weather, and Clouds
This section captures weather phenomena and cloud conditions. is reported in the initial data group as VV (00-99), coding distance from less than 0.1 km (00) to 50 km or more (90-99), following WMO Table 4701 increments. Present (ww 00-99 code) and past (W1W2 0-9 each) indicate current and recent conditions (e.g., , ). Cloud data includes total cover N (0-8 oktas, / none), and types/height via 8NhCLCMCH (Nh low cloud amount, CL low type, CM middle, CH high). Together, these provide context for atmospheric state. Section V: Wind, Temperature, and Dew Point
Concluding the core data, Section V details , air , and . Wind is encoded as Nddff (N total cloud if separate, dd direction 00-36 in 10° steps, 00 or / for calm/variable; ff speed 00-99 in units per i indicator: 0=m/s, 1=knots, etc.; 00fff for >99). is 1SnTTT (S sign 0/1, n precision 0/1, TTT in tenths °C, e.g., 10020 for +20.0°C). Dew point is 2SnTdTdTd (similar format, e.g., 20010 for +1.0°C). Variable winds use /ddff; gusts, if reported, use optional supplementary groups. This section refines hazard assessments and supports thermodynamic computations in forecasting models. Example
A sample SYNOP message: 12345 12000 8///// 27010 10250 20120 31015 40150 51/// 60000 03/// 8/000 (Station 12345, 12:00 UTC day 12, precip none, vis 8km, 270° 10 knots, temp +25.0°C, +12.0°C, station press 1015.0 hPa, MSL 1015.0 hPa, tendency +1.0 hPa/3h, no precip, present weather 03 (select), past none, no clouds).

Encoding Details

Standard Codes for Observations

The standard codes for observations in the SYNOP format (FM 12) are defined by the (WMO) to ensure uniform encoding of surface data across global reporting stations. These codes use numeric groups to represent key meteorological variables, allowing for compact transmission while maintaining precision for synoptic analysis. Pressure is encoded in dedicated groups within the message structure, typically as the 4PPPP group for sea-level pressure reduced to mean sea level in tenths of hectopascals (hPa), with the thousands digit omitted. For instance, the group 40120 represents 1012.0 hPa, where the leading 4 indicates sea-level reduction, and PPPP (0120) denotes the value in 0.1 hPa units. This encoding appears in Section 1 of the SYNOP message when pressure data is available, supporting the calculation of isobars in weather charts. Present weather is captured using the ww code (00-99) in groups such as 7wwWW, where ww describes the current or recent weather phenomena according to WMO Code Table 4677. Codes range from 00 (cloud development not observed or observable, sky clear) to 99 (unknown or unobservable), with specific values indicating precipitation intensity, type, and other conditions; for example, 61 denotes rain showers. Past weather is similarly coded in W1W2 (0-9 each), distinguishing phenomena over the past 24 hours from those in the three hours preceding the observation. Cloud-related codes include the total cloud amount N (0-8, representing oktas or eighths of sky coverage, with 0 for clear sky and 8 for overcast; / indicates sky obscured), and cloud types C for low (CL, 0-9), medium (CM, 0-9), and high (CH, 0-9) levels, drawn from WMO Code Tables 0509, 0513, 0515, and the International Cloud Atlas. For instance, CL=3 signifies stratocumulus clouds, while CH=7 indicates cirrostratus. These are reported in groups like 8NCLCMCH. Visibility is encoded as VV (two digits, 00-99) in groups such as iRixhVV, following WMO Code Table 4300, where values below 90 represent distances in hundreds of meters (e.g., 10 = 1,000 m or 1 km), and 90-99 indicate 10 km or greater, with 90 specifically for 10 km and 99 for 50 km or more. Reduced visibility due to specific causes, such as (visibility <1 km coded as 10-19) or (40-49), uses these ranges to highlight obstructions, aiding in and forecasting applications. Wind observations are detailed in the Nddff group of Section 1, where dd (00-99) specifies direction in tens of degrees from (e.g., 27 = 270° or westerly; 00 or /99 indicates calm or variable), and ff (00-99) gives speed in units determined by the iw indicator (0 or 2 for meters per second, 1 or 3 for s), with values up to 99; speeds exceeding 99 use three digits (fff). Calm winds are coded as 00, and light winds below 0.3 m/s (or 1 ) as 01. A sample decoded group from Section 3 (present and past ) is 76123, broken down as follows: 7 indicates the present weather group; ww=61 (rain showers); W1=2 ( during past 24 hours); W2=3 (showers of rain during past 3 hours). This group conveys recent activity without requiring full message context, illustrating how codes layer to describe temporal changes in weather conditions.

Supplementary Codes and Variations

In SYNOP reports, supplementary codes encompass optional groups that extend the core observational data, enabling more detailed or regionally relevant meteorological information while maintaining international compatibility. These groups are typically included after the mandatory sections and are used at the discretion of reporting stations, particularly for enhanced climate monitoring or specialized applications. The (WMO) defines these in its Manual on Codes to ensure consistency, with flexibility for national or regional adaptations. Section VI focuses on supplementary details for , , and station-level , providing not always captured in the primary groups. It includes air temperatures recorded over the preceding 12 or 24 hours, encoded as signed values in tenths of degrees (e.g., using Code Table 3845 for sign and magnitude, where values range from -99 to +60°C). Relative is reported in percent (0-100%), often as an optional group like 29UUU, reflecting conditions at the time of observation or averaged over a period. Station supplements, such as variations from sea-level equivalents, may also appear here for high-elevation sites, aiding in local tendency analysis. These elements enhance the utility of SYNOP for diurnal climate studies without overloading routine transmissions. Section VII addresses supplementary parameters and solar radiation metrics, offering insights into extended wind profiles or insolation. Additional data might include gust speeds exceeding mean values by more than 5 m/s, coded in knots or m/s (e.g., via group 0Gfmfmf, with fmfmf in tenths of m/s), or directional variations over recent hours. Duration of sunshine is quantified in hours, typically over the past 24 hours, using codes like 55SSS (SSS in whole hours, up to 999) or integrated solar radiation in kJ/m² (e.g., 5FFFF). These codes support applications in energy balance assessments and , where prolonged exposure data informs solar forecasting models. Section VIII captures indicators for supplementary environmental phenomena, particularly those with broader geophysical implications. It includes codes for coverage and characteristics (e.g., using Code Table 5239 for concentration, bearing, and distance, such as 8ICEciSi biDizi), snow cover depth or water equivalent (e.g., 9SPSP for depth in cm), and volcanic ash presence affecting visibility or air quality (e.g., via group ww=95 or supplementary 8app2 indicators). These are regionally applied, such as in polar or volcanic-prone areas, to flag hazards without requiring full narrative descriptions. National variations in supplementary codes permit member states to incorporate country-specific data under WMO regulations (e.g., Regulation 12.2.3.3), provided they notify the WMO Secretariat and avoid conflicting with international groups. For instance, the adds details on intensity and size in optional groups (e.g., section 9 extensions using national tables for descriptors), enhancing domestic severe storm warnings. In , supplements for air quality metrics, such as particulate matter levels tied to visibility reductions, are included in some regional exchanges (e.g., via EUMETNET adaptations), supporting integrated . These variations are confined to non-mandatory sections like 5 or 9 to preserve global interoperability. Adaptations for automated weather stations (AWS) streamline supplementary codes by omitting manual interpretive elements, such as subjective weather remarks, and using solidi (/) for unmeasured parameters (e.g., /// for unavailable ). AWS reports often prioritize numeric like automated extremes and via sensors, with an indicator like ix=4 denoting automatic origin; precipitation trends in sections VI or VII may default to 000 if no gauge is present. This ensures compatibility with SYNOP while reducing transmission length for remote, unmanned sites.

Applications and Usage

Role in Weather Forecasting

SYNOP messages provide essential surface weather observations that are plotted on synoptic charts every six hours at 0000, 0600, 1200, and 1800 UTC, enabling meteorologists to identify large-scale features such as pressure highs and lows, fronts, and pressure gradients for synoptic-scale analysis. These plots, using standardized symbols for elements like , , and present , facilitate the visualization of patterns and support short- to medium-range over continental and global scales. In (NWP), SYNOP data are assimilated into global models such as the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System and the (GFS) to initialize forecast cycles, improving the accuracy of initial atmospheric states through variational techniques like 4D-Var. processes, including buddy-checking, compare observations against neighboring stations and model backgrounds to detect and reject outliers, ensuring reliable input with tolerances adjusted for local variability. SYNOP observations support real-time operational meteorology, including weather services through linkage with reports for enhanced at airports, issuance of severe weather warnings for phenomena like storms and floods, and ongoing monitoring by contributing to long-term surface datasets. Despite their value, SYNOP data exhibit a ground-based observational due to sparse station networks, particularly over oceans and remote areas, which is mitigated in modern by integrating them with and observations for a more complete three-dimensional picture.

Data Dissemination and International Standards

SYNOP data are primarily disseminated through the World Meteorological Organization's (WMO) Global Telecommunication System (GTS), which serves as the main telecommunication network (MTN) for the rapid exchange of meteorological information among National Meteorological Services (NMS) worldwide. The GTS facilitates the transmission of SYNOP messages using internet protocols such as the (FTP), enabling near-real-time distribution to support global monitoring and operations. This infrastructure ensures that surface synoptic observational data from land stations, ships, and buoys (using SYNOP and related codes) are shared efficiently across regional associations and international centers. Observations encoded in SYNOP format are transmitted at standardized synoptic times to maintain consistency in global data collection. Main synoptic hours occur at 00, 06, 12, and 18 UTC, with these transmissions prioritized for their comprehensive coverage of atmospheric conditions, while intermediate reports at 03, 09, 15, and 21 UTC provide supplementary updates where required. This schedule aligns with WMO regulations to synchronize data availability for models and synoptic analysis. International compliance for SYNOP dissemination is governed by the WMO Manual on Codes (WMO-No. 306), which specifies the formatting, content, and transmission standards for alphanumeric codes like SYNOP (FM-12) to ensure across member states. Since 2018, efforts to migrate SYNOP data to the Binary Universal Form for the Representation of meteorological data (BUFR) have accelerated to enhance efficiency, reduce message size, and improve automated processing, though traditional alphanumeric formats remain in use during the transition. As of 2025, several National Meteorological Services, including China's, have discontinued parallel SYNOP transmission, though alphanumeric formats persist in regions during the ongoing phase-out. NMS must adhere to these standards, including validation procedures to verify data accuracy, completeness, and timeliness before release onto the GTS, thereby upholding the reliability of global meteorological networks.

Datasets and Resources

Global SYNOP Archives

The Global SYNOP Archives encompass major repositories that compile historical and contemporary synoptic weather observations for scientific research, climate reanalysis, and long-term trend studies. A primary resource is the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Information (NCEI) Integrated Surface Database (ISD), which aggregates hourly and synoptic surface observations from over 35,000 stations worldwide, including SYNOP-formatted data primarily from 1948 to the present, though some records extend to 1901. This database draws from diverse sources such as national meteorological services and international exchanges, providing essential variables like , , , and in a standardized ASCII format suitable for global analysis. Another key archive is the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis dataset, which assimilates digitized SYNOP observations alongside other data into a consistent global atmospheric model spanning 1940 to the present on a 31 km grid with hourly resolution. ERA5 incorporates surface synoptic reports to produce reanalyzed estimates of variables such as air temperature and sea-level pressure, enabling the reconstruction of historical weather patterns where direct observations are sparse. These archives collectively cover more than 10,000 active and historical SYNOP stations globally, offering hourly and daily resolutions; they also include digitized telegraphic records dating back to the 1850s through integrations like the International Surface Pressure Databank (ISPD), which compiles over 200 million pressure observations from land and marine sources starting in 1768, many derived from early SYNOP-like telegrams. Data quality in these archives is enhanced through rigorous homogenization processes to address biases from instrument changes, station relocations, and observational inconsistencies. For instance, the ISPD employs duplicate removal algorithms and quality controls, including feedback from reanalysis projects like the 20th Century Reanalysis (20CR), to ensure reliable pressure records for climate applications. Similarly, ISD undergoes automated flagging for outliers and manual reviews to maintain homogeneity. In climate studies, these SYNOP archives facilitate the extension of observational records for , such as in temperature homogenization projects that blend synoptic messages with monthly summaries to fill gaps and reduce uncertainties in long-term series. For example, researchers have used archived SYNOP to extend European daily temperature records backward by incorporating sub-daily observations from the onward, improving the detection of warming trends and variability. Such efforts support broader initiatives like the International Surface Temperature Initiative, where homogenized SYNOP-derived contribute to global assessments of climate change impacts.

Access Methods and Modern Extensions

SYNOP data, essential for global weather monitoring, is accessible through several established portals that provide metadata, real-time observations, and historical archives. The World Meteorological Organization's (WMO) Observing Systems Capability Analysis and Review (OSCAR) tool offers comprehensive metadata for surface observation stations, including those reporting in SYNOP format, via a RESTful API that delivers records in JSON format for querying station details, capabilities, and geographical coverage. For real-time access, the Synoptic Data Weather API aggregates surface observations from over 170,000 stations across more than 100 countries, enabling programmatic pulls of current and recent data in structured formats suitable for integration with SYNOP-derived variables like temperature, pressure, and wind. Open historical datasets, such as the National Oceanic and Atmospheric Administration's (NOAA) Integrated Surface Database (ISD), compile global hourly and synoptic surface observations from numerous sources into a unified ASCII format, facilitating research and analysis of long-term SYNOP records. Modern extensions to SYNOP enhance interoperability and efficiency, particularly in specialized domains like and data encoding. The ICAO Meteorological Information Exchange Model (IWXXM), an XML-based standard developed by WMO and ICAO, integrates surface weather reports—including elements from SYNOP—for use, allowing translation of traditional alphanumeric codes into machine-readable formats that support automated processing in systems. In parallel, WMO's ongoing transition to Binary Universal Form for the Representation of meteorological data (BUFR) in the 2020s supplements legacy SYNOP text codes, with full replacement pursued where feasible to improve data compression and global exchange; for example, the announced plans in 2019 to cease parallel dissemination of traditional SYNOP formats in favor of BUFR, reflecting broader adoption trends as of 2025. Emerging AI techniques are also being explored to automate decoding of legacy SYNOP archives, though widespread adoption remains limited to research applications like in synoptic weather maps. Several open-source tools facilitate parsing and visualization of SYNOP data for developers and researchers. The python_synop library, available on GitHub, provides a Python package for decoding SYNOP reports according to WMO specifications, converting text messages into structured dictionaries for easy manipulation and analysis. For visualization, the MetPy Python library supports plotting station-based observations, such as wind barbs, temperature, and pressure symbols, using data parsed from formats compatible with SYNOP, as demonstrated in its station plot examples that render synoptic-style maps from real-time feeds. Despite these advancements, challenges persist in SYNOP data coverage, particularly in developing regions where sparse networks create observational gaps that hinder accurate . WMO initiatives, highlighted at COP29 in , emphasize satellite-based augmentation—such as integrating geostationary and polar-orbiting imagery with surface reports—to bridge these disparities, aiming to expand the Global Basic Observing Network (GBON) and ensure equitable access to essential climate variables in underserved areas.

References

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