Glossary - Spire Weather API Documentation

Key terms and definitions used throughout the Spire Weather API documentation.

AGL: Above Ground Level. A vertical reference indicating height measured from the earth’s surface. Many surface and near-surface variables (e.g., 2-meter temperature, 10-meter wind) use AGL as their vertical datum.
API Key: A unique authentication token required for all Spire Weather API requests. Include it in the spire-api-key HTTP header.
ATCF ID: Automated Tropical Cyclone Forecast identifier. Format: BBXXYYYY where BB=basin, XX=cyclone number, YYYY=year. Example: AL022024.
Archive Data: Historical weather data from past forecast runs, retrieved through asynchronous job requests.
Base URL: The root URL for all API endpoints: https://api.wx.spire.com
Bundle: A collection of related weather variables grouped by use case (e.g., basic, maritime, agricultural). Multiple bundles can be requested in a single API call.
CAPE: Convective Available Potential Energy. A measure of atmospheric instability (J/kg) indicating the energy available for thunderstorm updrafts. Higher values suggest greater severe weather potential.
CF Conventions: Climate and Forecast metadata conventions, a standard for describing earth science data. Spire API field names follow these conventions for interoperability.
CIN: Convective Inhibition. A measure of atmospheric stability (J/kg) representing the energy barrier that must be overcome before convection can begin. Higher values suppress thunderstorm development.
Cloud Ceiling: The height above ground of the base of the lowest cloud layer with greater than 50% coverage. A critical parameter for aviation operations, measured in meters (SI) or feet (US).
CONUS: Continental United States. Refers to the 48 contiguous U.S. states, used as a geographic coverage area for products such as lightning density data.
CPHC: Central Pacific Hurricane Center. The NWS office responsible for issuing tropical cyclone advisories in the central Pacific basin. One of the data sources for the Storm Tracks API.
Deterministic Forecast: A single forecast run producing one predicted outcome, as opposed to ensemble forecasts which produce a range of possibilities.
EPSG: European Petroleum Survey Group (now the IOGP Geomatics Committee). EPSG codes identify coordinate reference systems. For example, EPSG:4326 is the standard WGS 84 latitude/longitude system used in WMS requests.
Ensemble Forecast: A collection of multiple forecast runs with slightly varied initial conditions, used to estimate forecast uncertainty.
Field Mask: A filter specified in the X-Fields header to request only specific fields in the API response, reducing payload size.
Forecast Hour: The time offset from the issuance time, expressed in hours. Appears in file names as fHHH (e.g., f000 for analysis time, f006 for 6 hours after issuance). Also called lead hour.
Geopotential Height: The height of a pressure surface above mean sea level, adjusted for variations in gravitational acceleration. Measured in geopotential meters (gpm) and used extensively in upper-air analysis.
GHI: Global Horizontal Irradiance. The total shortwave radiation received by a horizontal surface at the earth’s surface (W/m²). A key input for solar energy forecasting.
DNI: Direct Normal Irradiance. The shortwave radiation per unit area measured on a surface perpendicular to the sun’s rays (W/m²). Important for concentrating solar power systems.
GRIB2: GRIdded Binary Edition 2, a WMO standard format for storing and transmitting gridded meteorological data. Spire forecast files are provided in this format.
hPa: Hectopascal. A unit of atmospheric pressure equal to one millibar (mb). Standard sea-level pressure is approximately 1013.25 hPa. Used in the Storm Tracks API for cyclone central pressure and in upper-air data for isobaric level identification.
ICAO Code: International Civil Aviation Organization airport identifier. A 4-letter code like KJFK (New York JFK) used in optimized forecast requests.
ISO 8601: An international standard for representing dates, times, and durations as strings. The API uses ISO 8601 format for all timestamp parameters (e.g., 2024-01-15T00:00:00Z) and time ranges (e.g., 2024-01-01T00:00:00/2024-01-31T00:00:00).
Isobaric Level: A surface of constant atmospheric pressure, used as vertical coordinates for upper-air data. Spire provides data on standard isobaric levels from 1000 hPa (near surface) to 1 hPa (upper stratosphere).
Issuance Time: The timestamp when a forecast model run was initiated. Also called the reference time or analysis time.
JTWC: Joint Typhoon Warning Center. A U.S. military agency that issues tropical cyclone advisories for the western Pacific, Indian Ocean, and Southern Hemisphere. One of the data sources for the Storm Tracks API.
KMZ: Keyhole Markup Language Zipped. A compressed file format used by Google Earth for geographic data visualization. The Storm Tracks API can return storm data in KMZ format for viewing in Google Earth Pro.
Knot: A unit of speed equal to one nautical mile per hour (approximately 0.514 m/s or 1.151 mph). Used in the Storm Tracks API for wind speeds and storm movement. Abbreviated as “kt” or “kts”.
Lead Time: The time interval between when a forecast is issued and when it is valid. For example, a forecast issued at 00Z valid at 06Z has a 6-hour lead time.
Model Run: A single execution of a numerical weather prediction model, starting from a set of initial conditions at a specific issuance time. Spire’s global model runs 4 times per day (00Z, 06Z, 12Z, 18Z).
MSL: Mean Sea Level. A vertical reference datum representing the average ocean surface height. Used as the reference for tidal heights, sea-level pressure, and some elevation measurements.
NetCDF: Network Common Data Form, a self-describing binary file format for array-oriented scientific data. Used for some Spire data products.
NHC: National Hurricane Center. The U.S. National Weather Service office responsible for issuing tropical cyclone advisories in the Atlantic and eastern Pacific basins. One of the data sources for the Storm Tracks API.
NOAA: National Oceanic and Atmospheric Administration. The U.S. federal agency that oversees the National Weather Service (NWS), National Hurricane Center (NHC), and other offices that produce weather observations and forecasts.
NWP: Numerical Weather Prediction. The use of mathematical models of the atmosphere to forecast weather. Spire runs its own NWP models to produce forecast data.
NWS: National Weather Service. The U.S. federal agency responsible for weather forecasting and issuing warnings. The NHC and CPHC are offices within the NWS.
OGC: Open Geospatial Consortium. An international organization that develops open standards for geospatial data. Spire’s WMS endpoint implements the OGC Web Map Service standard.
Optimized Point Forecast: A post-processed, bias-corrected forecast for named locations such as airports, ports, and weather stations. Updated hourly and accessed via the /forecast/point/optimized endpoint using ICAO, WMO, or UN/LOCODE identifiers.
POA: Plane of Array Irradiance. The solar radiation incident on a tilted photovoltaic panel surface (W/m²). Accounts for panel tilt and orientation, making it the most directly relevant irradiance measure for solar farm output.
Point Forecast: Weather data for a specific latitude/longitude coordinate, returned as JSON. Contrast with file products which provide gridded coverage.
Power Forecast: A specialized forecast predicting aggregated power generation (in megawatts) from wind or solar assets over a geographic region. Available for select European regions via the /forecast/power endpoint.
Redirect: HTTP 302 response used for file downloads. Clients must follow redirects (curl -L flag) to retrieve actual file content.
Reflectivity: A measure of how strongly precipitation scatters radar energy, expressed in dBZ (decibels relative to Z). Composite reflectivity is the maximum value in the vertical column, used to identify precipitation intensity and type.
Resolution: The grid spacing of forecast data. Expressed as degrees (e.g., 0.25° ≈ 28 km at the equator). Higher resolution means finer detail.
Route Forecast: Weather predictions along a specified path with timestamps, used for shipping, aviation, and travel planning.
SI Units: International System of Units, the default unit system for API responses. Temperature in Kelvin, wind in m/s, pressure in Pa.
Soil Moisture: The water content in soil, expressed as volumetric water content (m³/m³). Important for agriculture and hydrology.
Significant Wave Height: The average height of the highest one-third of waves, denoted Hs or H1/3. A key parameter for maritime applications.
Spin-up Time: The initial period of a model run during which certain derived fields (e.g., accumulated precipitation, composite reflectivity, total cloud cover) may output zeros or unreliable values as the model’s internal processes stabilize. Typically affects only the analysis time (f000).
SRFS: Spire Regional Forecast System. A high-resolution (~3 km) regional forecast model providing finer spatial detail over supported regions. File names use the srfs product identifier.
Storm-Relative Helicity: A measure of the potential for a thunderstorm updraft to rotate (m²/s²), calculated over a specified depth layer (typically 0–3 km AGL). Higher values indicate greater tornado potential.
Swell: Ocean waves that have traveled beyond the area where the wind generated them. Swell waves are longer-period and more regular than locally generated wind waves. The API provides swell height, direction, and period separately from wind wave components.
Tidal Datum: A reference surface for measuring tidal heights. Mean Sea Level (MSL) is the default datum for Spire tidal data.
Time Bundle: A query parameter (time_bundle) that controls the temporal resolution and forecast horizon of returned data. Options include hourly (1h steps, 2-day range), 3_hourly, 6_hourly, hourly_6day, and all. Available time bundles depend on your subscription tier.
UN/LOCODE: United Nations Code for Trade and Transport Locations. Identifies ports and cities globally (e.g., USNYC for New York).
US Units: United States customary units. Temperature in Fahrenheit, wind in mph, pressure in inHg. Request with unit_system=us.
UTC: Coordinated Universal Time, the primary time standard for API timestamps. Also written as “Z” (Zulu time).
Valid Time: The future timestamp for which a forecast prediction is made. The difference between valid time and issuance time is the forecast lead time.
Vorticity: A measure of the local rotation (spin) of the atmosphere (s⁻¹). Absolute vorticity, available on isobaric levels, combines the rotation due to wind patterns with the Earth’s rotation.
Waypoint: A geographic point along a route, defined by latitude, longitude, and a timestamp. Route forecast and archive endpoints accept up to 120 waypoints per request.
Wind Radii: The extent of winds of a given speed around a tropical cyclone, measured in nautical miles for each quadrant (NE, SE, NW, SW). The Storm Tracks API provides wind radii at the 34-knot, 50-knot, and 64-knot thresholds.
Wind Shear: The change in wind speed and/or direction over a specified vertical depth. Deep-layer shear (0–6 km) is a key parameter for severe weather forecasting, as it influences thunderstorm organization and longevity.
WMO ID: World Meteorological Organization station identifier. A 5-digit number identifying synoptic weather stations.
WMS: Web Map Service. An OGC standard protocol for serving georeferenced map images over the internet. Spire’s WMS endpoint (/ows/wms) provides weather data as map layers that can be consumed by GIS applications such as Leaflet, OpenLayers, and QGIS.