A
APRS-IS (Automatic Packet Reporting System — Internet Service): The internet-based backbone of the CWOP network. Your station sends weather data packets to APRS-IS servers using your call sign as the identifier. No radio license or equipment required for weather stations using the internet pathway.
ASOS (Automated Surface Observing System): The network of official NWS and FAA weather stations at airports across the US. These are the reference stations most commonly used to verify home station accuracy. Data is available in real time at aviationweather.gov as METAR reports.
Altimeter Setting (QNH): Barometric pressure adjusted to sea level using a standard atmospheric model. This is what weather apps report and what you should calibrate your station to display. Found in airport METAR reports as the "A" value (e.g., A2992 = 29.92 inHg).
C
CWOP (Citizen Weather Observer Program): A joint NOAA/NWS program for amateur weather observers. Registered stations receive a call sign (CW/EW prefix) and submit observations to MADIS for use in NWS forecast models. Free to register; see our CWOP registration guide.
CoCoRaHS (Community Collaborative Rain Hail and Snow Network): A network of volunteer precipitation observers using standardized manual gauges. CoCoRaHS data is the best local reference for verifying rain gauge accuracy.
D
Dew Point: The temperature to which air must be cooled for water vapor to condense. A direct measure of actual atmospheric moisture content — more useful for comfort assessment and frost prediction than relative humidity. See our dew point guide.
Daytime Radiation Error: The artificial temperature inflation caused by solar heating of a passive radiation shield. The primary cause of afternoon temperature readings that run 5–15°F above actual air temperature on sunny days.
G
Gateway (Ecowitt GW1100, GW2000): The central hub of an Ecowitt weather station system. The gateway receives wireless data from all sensors, connects to your home Wi-Fi, and uploads to cloud services and weather networks. Configured via the WSView Plus app.
H
Heat Index: An apparent temperature calculation combining air temperature and relative humidity. Reflects the perceived temperature when humidity reduces the effectiveness of evaporative cooling. Most relevant above 80°F (27°C).
M
MADIS (Meteorological Assimilation Data Ingest System): NOAA's system for ingesting observations from multiple sources including CWOP, ASOS, and other networks. NWS forecasters access MADIS to view your station's data.
METAR: Standard aviation weather report format issued hourly (or more frequently) by ASOS stations. Contains temperature, dew point, wind, visibility, cloud cover, and altimeter setting.
P
Passive Radiation Shield: The louvered white plastic housing that protects a temperature/humidity sensor from direct solar radiation while allowing airflow. Relies on natural wind for ventilation — in calm conditions can accumulate heat error. Contrast with aspirated radiation shield (fan-ventilated).
PWS (Personal Weather Station): Any privately operated weather station that uploads data to a public network. The term is used by Weather Underground, Ecowitt, and CWOP to distinguish amateur citizen science stations from official NWS/FAA stations.
R
Radiation Shield: See Passive Radiation Shield. Also called a "stevenson screen" in reference to the traditional louvered wooden box used at official stations.
RSSI (Received Signal Strength Indicator): A measure of Wi-Fi signal strength, expressed in negative dBm values. -50 dBm = excellent; -70 dBm = fair; below -80 dBm = unreliable for continuous upload. Check in WSView Plus under device info.
S
Sea Level Pressure: Barometric pressure corrected to what it would be at sea level using a standard atmosphere formula. Allows pressure comparison across locations at different elevations. Calculated from station pressure using elevation.
T
Tipping Bucket Rain Gauge: The standard rain gauge type in consumer weather stations. Rain collects in a funnel and drains into one of two small buckets on a pivot. Each 0.01 inch of rain fills and tips the bucket, sending a signal to the station electronics.
W
WMO (World Meteorological Organization): The UN agency that establishes international standards for weather observation. WMO siting criteria define station classes (1–5) based on placement quality, from ideal open terrain to heavily obstructed urban locations.
WeeWX: Open-source weather station software that runs on Linux (including Raspberry Pi). Records all station data locally, generates HTML reports, and can upload to multiple networks simultaneously. See our WeeWX setup guide.
WSView Plus: Ecowitt's free mobile app (iOS and Android) for configuring and monitoring Ecowitt gateway devices. Used for Wi-Fi setup, sensor calibration, upload service configuration, and firmware updates.
WU / Weather Underground: The largest public personal weather station network, owned by IBM's The Weather Company. Your station data appears on the WU map and is accessible via API. Registration is free; see our WU setup guide.