Glossary · StormWorn

Enhanced Fujita scale (EF): The scale used to rate U.S. tornadoes since February 1, 2007, from EF0 to EF5, based on damage to 28 types of structures and vegetation. EF0 winds are estimated at 65–85 mph and EF5 at more than 200 mph. EF-U means the tornado could not be rated, usually because it struck nothing that could show its strength.
Fujita scale (F): The scale used from 1971 to January 2007, and applied to earlier tornadoes from historical accounts, from F0 to F5. Ratings on the two scales are broadly comparable.
Path length and width: The distance a tornado travelled on the ground, in miles, and its maximum width, in yards, as surveyed by the National Weather Service.
Saffir-Simpson Hurricane Wind Scale: The 1-to-5 rating of hurricanes by maximum sustained wind: Category 1, 74–95 mph; Category 2, 96–110 mph; Category 3, 111–129 mph; Category 4, 130–156 mph; Category 5, 157 mph or more. Categories 3 to 5 are major hurricanes.
Tropical storm: A tropical cyclone with maximum sustained winds of 39–73 mph (34–63 knots). Below that it is a tropical depression.
Post-tropical cyclone: A former tropical cyclone that has lost its tropical characteristics, often while still producing strong winds and heavy rain. Listed on place pages but not counted.
Closest approach: The shortest distance between a storm’s track and the place, measured to the county boundary or the city centre, using positions interpolated hourly.
HURDAT2: The National Hurricane Center’s official database of tropical cyclone positions and intensities, revised as historical storms are re-analysed.
Storm Events Database: NOAA’s official record of significant weather events, with dates, locations, casualties, damage estimates and narratives written by National Weather Service offices.
Forecast zone: An area, often one county or part of one, used by the National Weather Service for forecasts and warnings. Many winter, heat and cold events are filed by zone.
MTBS: Monitoring Trends in Burn Severity, a USGS and U.S. Forest Service program that maps large fires from Landsat imagery since 1984.
Prescribed burn: A fire set on purpose by land managers to reduce fuel. Excluded from wildfire totals.
National Flood Insurance Program (NFIP): The federal program, run by FEMA, that writes most U.S. residential flood insurance. Its claims show insured flood losses, not all flood losses.
Flood stage: The water level at a river gauge at which flooding begins to affect people or property, set by the National Weather Service with local officials. Minor, moderate and major flood stages mark rising levels of impact; action stage is the level at which officials prepare.
Crest: The highest level a river reaches during a flood at a gauge, in feet on the gauge’s own scale. Stages from different gauges are not comparable; the height above each gauge’s flood stage is.
Palmer Drought Severity Index (PDSI): A monthly index of the long-term balance between moisture supply and demand, from temperature and precipitation, relative to a place’s own climate. Zero is normal, −2 moderate drought, −3 severe, −4 extreme; positive values are wet.
Hazard Mapping System (HMS): NOAA’s daily analysis of fires and smoke, in which analysts trace smoke plumes seen in satellite imagery and class them light, medium or heavy.
Magnitude: A measure of the size of an earthquake from its recorded seismic waves. Each whole number is about 32 times more energy than the one below. Catalogues mix several scales (moment, local, body-wave), which agree only roughly.
Aftershock: A smaller earthquake that follows a larger one on or near the same fault. Earthquake pages count those of magnitude 2.5 or more in the 30 days after, within the rupture length expected for the main shock.
Epicentre: The point on the surface directly above where an earthquake began. Distances on StormWorn are measured from it.
Major disaster declaration (DR): A presidential declaration that unlocks federal recovery assistance after a disaster beyond state and local capacity.
Emergency declaration (EM): A presidential declaration of narrower scope, typically for protective measures before or during an event.
Fire management declaration (FM): Federal assistance for fighting a wildfire that threatens to become a major disaster.
First-order station: A weather station staffed or maintained by the National Weather Service or FAA, usually at an airport, with the most consistent records.
Threaded record: A city’s official climate record joined from several stations over time, such as a downtown site followed by an airport. StormWorn uses one station per place, so its records can differ.
Percentile: The share of counties with a lower value. A county at the 90th percentile ranks above 90 percent of the counties compared.
Population centre: The point where a county’s population would balance on a weightless map, from the 2020 Census. Used to choose a representative weather station.