Automatic Weather Stations (AWS) form the backbone of modern surface observational networks. Whether deployed for aviation, agriculture, flood warning, or climate monitoring, their data are only as good as the standards they adhere to. The World Meteorological Organization's Guide to Instruments and Methods of Observation (WMO-No. 8, also known as the CIMO Guide) is the internationally recognised reference for meteorological measurement practice.
This article provides a practical summary of the key WMO requirements that affect AWS design, siting, sensor selection, and data quality assurance. If you are specifying, deploying, or maintaining automatic weather stations, this is your essential checklist.
Siting and Exposure Requirements
The WMO places great emphasis on siting because even the best sensors will produce unrepresentative data if poorly exposed. Key requirements include:
- Temperature and humidity: Sensors should be housed in naturally ventilated or aspirated radiation shields, mounted at 1.25–2.0 m above ground level, over a surface representative of the surrounding area.
- Wind: Anemometers should be mounted at 10 m above ground, with no significant obstacles within a radius of at least 10 times the obstacle height.
- Precipitation: Rain gauges should be sited on flat terrain with a clear fetch. The angle from the gauge orifice to the top of surrounding obstacles should not exceed 30°. Wind shields (e.g., Alter shields) are recommended for catch-deficiency reduction.
- Radiation: Pyranometers require an unobstructed horizon — no object above 5° elevation in any direction.
"The exposure of instruments is just as important as their accuracy. A precise sensor in a poor location will produce misleading data." — WMO CIMO Guide, Chapter 1
Sensor Specifications and Measurement Uncertainty
The CIMO Guide defines required measurement ranges, resolutions, and achievable uncertainties for all standard meteorological parameters. Here are key targets:
Temperature
- Range: −80 °C to +60 °C
- Resolution: 0.1 K
- Achievable uncertainty: 0.1 K (operational), 0.04 K (reference)
- Sensor types: PRT (Pt100/Pt1000), thermistors
- Time constant: 20 s maximum for standard applications
Relative Humidity
- Range: 0–100% RH
- Resolution: 1% RH
- Achievable uncertainty: 3% RH (operational), 1% RH (reference)
- Sensor types: Capacitive polymer sensors dominate modern AWS
Atmospheric Pressure
- Range: 500–1080 hPa
- Resolution: 0.1 hPa
- Achievable uncertainty: 0.1 hPa
- Sensor types: Aneroid, resonant silicon, MEMS
Wind Speed and Direction
- Speed range: 0.5–75 m/s
- Speed uncertainty: 0.5 m/s (below 5 m/s), 10% (above 5 m/s)
- Direction uncertainty: 5°
- Sensor types: Cup anemometers, ultrasonic anemometers, propeller-vane systems
- Averaging: 2-min mean for sustained; 3-s gust for peak wind
Data Acquisition and Quality Control
Modern AWS dataloggers must be capable of:
- High-frequency sampling (typically 1 Hz or faster for wind)
- On-board computation of 1-min, 10-min, and 60-min aggregates
- Real-time QC flags using range checks, rate-of-change checks, and internal consistency tests
- Timestamping to UTC with ≤1 s accuracy (GNSS synchronisation recommended)
- Local data storage with sufficient capacity for at least 30 days of autonomous operation
The WMO OSCAR/Surface database provides metadata standards for documenting station characteristics, ensuring data can be properly interpreted and homogenised by downstream users.
Calibration and Maintenance
The CIMO Guide mandates regular calibration traceable to national and international standards. Key points:
- Temperature: Field checks against a travelling standard every 6–12 months; laboratory calibration every 2–5 years
- Humidity: Saturated salt solution checks quarterly; full calibration annually
- Pressure: Comparison with a digital transfer standard every 6 months
- Wind: Annual check of threshold speed, distance constant, and directional alignment
- Precipitation: Volume/mass check and tipping mechanism inspection every 6 months
Preventive maintenance schedules — including cleaning radiation shields, checking cable integrity, and verifying power supply health — are essential for maintaining data continuity.
Achieving Compliance
Full WMO compliance requires attention to four pillars:
- Instrument quality: Sensors meeting CIMO performance specs with documented traceability
- Siting: Exposure that meets classification criteria (WMO siting classification 1–5)
- Data handling: Sampling, averaging, and QC in line with CIMO recommendations
- Metadata: Complete station documentation in WIGOS/OSCAR format
If you are planning a new AWS deployment or auditing an existing network against WMO standards, the investment in compliance pays dividends in data credibility, interoperability, and long-term scientific value.