Weather and Nature

Environment and live-data glossary

The terms on our earthquake, air-quality, UV and aurora pages, defined one by one, naming the agency that publishes each scale.

15 terms · The definitions are ours; where a term has an official definition, the source is named.

A seismograph on display at a physics institute, with its pendulum and recording drum
A seismograph: it is the network of these instruments that produces every earthquake magnitude. · Photo: Lesless — CC BY-SA 4.0 · file page on Wikimedia Commons
Magnitude
A measure of the size of an earthquake at its source, on a logarithmic scale: each whole point multiplies the energy released by about 31.6. A magnitude 7 event releases a thousand times more energy than a magnitude 5, not 1.4 times.
Standard definition by U.S. Geological Survey (USGS) — ANSS ComCat
Focal depth
How many kilometres below the surface the rupture began. The USGS calls it shallow up to 70 km, intermediate from 70 to 300 km and deep beyond that. A shallow quake shakes the surface far harder than a deep one of the same magnitude.
Standard definition by U.S. Geological Survey (USGS) — ANSS ComCat
Richter and moment magnitude
The Richter scale, from 1935, was calibrated for Californian seismographs and saturates: above 7 it stops separating events. Agencies now publish moment magnitude (Mw), computed from the seismic moment — fault area, slip and rock rigidity. That is what the news still calls "the Richter scale".
Standard definition by U.S. Geological Survey (USGS) — ANSS ComCat
AQI
The United States Air Quality Index, 0 to 500, which converts each pollutant concentration into a single score and reports the worst one. Up to 50 is good, above 100 it starts to affect sensitive groups, above 150 it affects everyone. The score is not linear in concentration: it follows breakpoints set by the EPA.
Standard definition by U.S. EPA — AQI Technical Assistance Document
UV index
An open-ended scale starting at 0 that measures the strength of ultraviolet radiation able to redden skin. From 3 to 5 protection is already advised, 8 to 10 is very high and above 11 is extreme. It depends on how high the Sun is, on the ozone layer, on altitude and on cloud.
Standard definition by WHO / WMO — Global Solar UV Index
Kp index
A 0-to-9 number summarising the disturbance of Earth’s magnetic field over a three-hour window, measured by a network of observatories. Kp 5 is the weakest geomagnetic storm on NOAA’s G scale, and the higher the Kp, the further from the pole the aurora can be seen.
Standard definition by NOAA Space Weather Prediction Center
Preliminary data
A measurement published as soon as it is collected, before the agency’s final review. It can be corrected or withdrawn: earthquake magnitudes shift in the first hours, air-quality readings are revised weeks later. Use it to plan your day, not for regulatory or legal purposes.
Epicentre
The point on the surface directly above the earthquake’s focus, where the rupture began. The place a catalogue names is the nearest geographical reference to the epicentre, not necessarily the worst-hit town: that depends on depth and on ground type.
Aftershock
A smaller earthquake following a larger one on the same fault, while the rock settles into its new distribution of stress. Aftershocks are most frequent in the first hours and can continue for months, always below the magnitude of the main event.
PM2.5
Particulate matter under 2.5 micrometres across, fine enough to reach the depths of the lung. It is the pollutant that most often sets the AQI, and it comes from burning: diesel engines, fire, wood stoves, industry.
Standard definition by U.S. EPA — AQI Technical Assistance Document
Ground-level ozone
Ozone formed near the ground when sunlight reacts with exhaust gases and solvents. It is not the ozone layer, which sits kilometres up and protects us: this one irritates the airways and rises on clear, hot, windless days.
Standard definition by U.S. EPA — AQI Technical Assistance Document
Ultraviolet radiation
The invisible part of sunlight with wavelengths below violet. UVB burns skin and is largely filtered by the ozone layer; UVA goes deeper, passes through glass and ages skin. The UV index sums both up by their reddening effect.
Standard definition by WHO / WMO — Global Solar UV Index
Saffir-Simpson category
A 1-to-5 scale that classifies a hurricane by its maximum sustained wind. From category 3 up a hurricane counts as "major", with significant structural damage expected; the scale ignores rain, size and storm surge, which can be deadly even at a low category.
Standard definition by NHC — National Hurricane Center (Saffir-Simpson Hurricane Wind Scale)
FRP (fire radiative power)
The energy released by a fire detection, in megawatts, measured by satellite from the infrared brightness of the burn. A higher FRP means a more intense fire in that satellite pixel, not necessarily a bigger one: a small, very hot fire can outscore a large, weak one.
Standard definition by NASA FIRMS — Fire Information for Resource Management System
Significant wave height
The average of the highest third of waves measured by a buoy over a time window, not the single tallest wave seen. In practice, occasional individual waves reach nearly double that value, which is why surfers and sailors treat significant height as a floor, not a ceiling.
Standard definition by NOAA NDBC — National Data Buoy Center