METAR (Meteorological Aerodrome Report) is the standard format for aviation weather observations, issued by airports and automated weather stations worldwide. CWOP (Citizen Weather Observer Program) allows hobby weather station operators to contribute their data to a shared network, where it can be accessed by the National Weather Service and researchers. Understanding both systems helps you calibrate your station, validate your readings, and contribute meaningfully to the observation infrastructure. The NOAA Automated Surface Observing Systems (ASOS) site provides details on the professional stations that generate most METAR reports in the United States. This guide connects to the calibration practices in Station Data Sanity Checks and the siting standards in Observation Standards.

METAR Format Explained

A METAR report is a single line of encoded text. Here is a sample and its breakdown:

METAR CYUL 051800Z 27015G25KT 15SM FEW040 SCT250 22/12 A3002 RMK CU2CI1 SLP170

• CYUL — ICAO station identifier (Montréal–Trudeau)
• 051800Z — Day 05, 18:00 UTC
• 27015G25KT — Wind from 270° at 15 knots, gusting 25 knots
• 15SM — Visibility 15 statute miles
• FEW040 SCT250 — Few clouds at 4,000 ft, scattered at 25,000 ft
• 22/12 — Temperature 22°C, dewpoint 12°C
• A3002 — Altimeter setting 30.02 inHg
• SLP170 — Sea-level pressure 1017.0 hPa

Key Fields for Station Operators

The most useful METAR fields for validating your own station are temperature, dewpoint, pressure (SLP value), and wind. Compare your readings to the nearest airport METAR at the same time. Expect some systematic differences — your station is at a different location, altitude, and microclimate — but large discrepancies indicate calibration or siting problems.

CWOP and APRS-IS

CWOP uses APRS-IS (Automatic Packet Reporting System – Internet Service) to receive and distribute weather data. Your station software formats the observation as an APRS weather packet and sends it to an APRS-IS server. The data then flows to the Meteorological Assimilation Data Ingest System (MADIS) at NOAA, where it undergoes quality control before being made available to forecast models and researchers.

Registration

CWOP registration assigns you a station identifier (format: CW#### for US stations, or your amateur radio callsign). Registration requires your station coordinates, elevation, and equipment description. Once registered, your station appears on the CWOP station map and your data is archived.

Data Submission

Most station software (Weather Display, weewx, WeeWX, Cumulus MX) has built-in CWOP support. Configuration typically requires:

• Your CWOP station identifier
• An APRS-IS server address (e.g., cwop.aprs.net:14580)
• Your station latitude, longitude, and altitude
• Upload interval (typically 5–10 minutes)

Using METAR for Calibration

The nearest airport METAR is your most accessible calibration reference. Compare readings over several days under different conditions:

• Pressure: should track within 1–2 hPa after altitude correction. Consistent offset suggests incorrect station altitude or sensor drift.
• Temperature: expect 1–3°C difference due to microclimate. Larger gaps, especially on sunny afternoons, suggest poor radiation shielding on your sensor.
• Humidity: airports often report lower humidity because stations are on open tarmac areas. Your garden sensor may legitimately read higher.
• Wind: airport anemometers are at 10 m over flat terrain. Your readings will almost always be lower unless you have an exposed hilltop site.

Troubleshooting Matrix

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