Natural Disaster Glossary

Plain-language definitions of the terms used across this site and in official disaster reports. Each definition is written to answer the question directly.

Earthquakes & Seismology

Magnitude: A logarithmic measure of the energy released by an earthquake at its source. Each whole-number increase represents roughly 32 times more energy. Modern agencies use moment magnitude (Mw), which replaced the Richter scale for large events.
Intensity: A measure of how strongly an earthquake is felt at a specific location, usually on the Modified Mercalli scale (I–XII). Unlike magnitude, intensity varies from place to place depending on distance, depth and local ground conditions.
Epicenter: The point on the Earth's surface directly above where an earthquake begins. The actual underground starting point is called the hypocenter or focus.
Aftershock: A smaller earthquake that follows a larger one in the same area as the crust adjusts to the main rupture. Aftershocks can continue for weeks to years and can collapse buildings damaged by the mainshock.
Foreshock: A smaller earthquake that precedes a larger one in the same location. Foreshocks can only be identified as such after the larger event occurs — they cannot be recognized in advance.
Seismic waves: Energy waves that travel through the Earth from an earthquake source. Fast P-waves arrive first, stronger S-waves second, and slower surface waves usually cause the most damage.
Liquefaction: A process during strong shaking where water-saturated sandy soil temporarily behaves like a liquid, causing buildings to sink or tilt. It devastated parts of Palu, Indonesia in 2018.
Fault: A fracture in the Earth's crust where blocks of rock move past each other. Most earthquakes occur on faults, and long plate-boundary faults like the Sagaing Fault or San Andreas Fault produce the largest ones.
Ring of Fire: A roughly 40,000 km horseshoe-shaped zone around the Pacific Ocean where most of the world's earthquakes (about 80–90%) and about 75% of active volcanoes occur, caused by subducting tectonic plates.

Tsunamis & Coastal Hazards

Tsunami: A series of ocean waves caused by sudden displacement of water, usually from an undersea earthquake, landslide or volcanic eruption. In deep water tsunamis travel at jet-aircraft speeds; near shore they slow and grow in height.
Drawback: The sudden withdrawal of the sea before a tsunami wave arrives, exposing the seafloor. It is a natural warning sign — anyone seeing it should move to high ground immediately.
Storm surge: An abnormal rise of seawater above the normal tide, pushed ashore by a storm's winds and low pressure. Surge — not wind — causes most tropical-cyclone deaths.
Run-up: The maximum height above sea level that tsunami water reaches on land. The 2011 Tōhoku tsunami produced run-ups approaching 40 metres in some bays.

Storms & Weather

Tropical cyclone: A rotating storm system with sustained winds above 119 km/h, formed over warm tropical oceans. It is called a hurricane in the Atlantic and Northeast Pacific, a typhoon in the Northwest Pacific, and a cyclone in the Indian Ocean and South Pacific.
Eye and eyewall: The eye is the calm, often clear centre of a tropical cyclone; the eyewall around it contains the storm's most violent winds. The calm eye is dangerous — winds return suddenly from the opposite direction as it passes.
Saffir-Simpson scale: A 1–5 category scale rating hurricane sustained wind speed, from Category 1 (119–153 km/h) to Category 5 (above 252 km/h). It does not measure storm surge, rainfall or size.
Tornado: A violently rotating column of air extending from a thunderstorm to the ground, rated EF0–EF5 on the Enhanced Fujita scale from observed damage. The strongest tornadoes exceed 320 km/h.
Supercell: A long-lived, rotating thunderstorm containing a mesocyclone. Supercells produce most strong tornadoes, giant hail and damaging straight-line winds.
Monsoon: A seasonal reversal of winds that brings a distinct rainy season, most famously across South Asia from June to September. Monsoon variability drives both floods and droughts across the region.
Cloudburst: An extremely intense, localized downpour — commonly defined in South Asia as more than 100 mm of rain within one hour over a small area — that triggers flash floods and landslides, especially in mountains.
Derecho: A widespread, long-lived windstorm associated with a fast-moving line of thunderstorms, producing hurricane-force straight-line winds along a path hundreds of kilometres long.
El Niño / La Niña (ENSO): A natural Pacific Ocean climate cycle. El Niño (unusually warm eastern Pacific) and La Niña (unusually cool) shift global rainfall and storm patterns, influencing floods, droughts and cyclone seasons worldwide.

Floods & Water

Flash flood: A rapid flood that develops within minutes to six hours of intense rainfall (or a dam/levee failure), often in steep terrain, urban areas or dry channels. Flash floods are the deadliest type of flooding.
Riverine flood: Flooding that occurs when a river overflows its banks after prolonged rain or snowmelt, typically developing over days and covering large areas — as in Pakistan's 2010 and 2022 superfloods.
GLOF (Glacial Lake Outburst Flood): A sudden flood released when a lake dammed by a glacier or moraine fails. A growing hazard in the Himalaya, Karakoram and Andes as glaciers retreat.
Floodplain: The low-lying land beside a river that naturally floods when the river overflows. Building on floodplains is a leading cause of rising flood losses worldwide.
100-year flood: A flood with a 1% chance of occurring in any given year — not one that happens once per century. Back-to-back "100-year floods" are statistically possible and increasingly observed.

Volcanoes

Pyroclastic flow: A fast-moving avalanche of hot gas, ash and rock (often over 100 km/h and several hundred °C) that races down a volcano's slopes. It is the deadliest volcanic hazard and cannot be outrun.
Lahar: A volcanic mudflow of ash, rock and water that flows down river valleys, sometimes triggered by rain long after an eruption. The 1985 Armero lahar in Colombia killed about 23,000 people.
VEI (Volcanic Explosivity Index): A 0–8 scale for eruption size based on the volume of erupted material and plume height. Each step is roughly 10 times larger; Tambora 1815 (VEI 7) was the largest in recorded history.
Volcanic ash: Pulverized rock and glass fragments ejected by eruptions — abrasive, conductive and heavy. It damages lungs, engines and roofs, and is not like soft fireplace ash.

Landslides, Drought & Temperature

Landslide: The downslope movement of rock, soil and debris under gravity, usually triggered by intense rain, earthquakes or slope modification. Debris flows are the fast, water-charged type.
Avalanche: A rapid flow of snow down a slope, most often triggered on 30–45° slopes during or after storms, wind-loading or rapid warming — frequently by the victims themselves.
Drought: An extended period of below-normal precipitation causing water shortage. Unlike sudden disasters, drought develops over months and is declared using rainfall, soil-moisture and reservoir indicators.
Heatwave: A prolonged period of abnormally hot weather relative to the local climate. Heatwaves are among the deadliest natural hazards, killing mainly through heat stroke and strain on hearts — often indoors and unnoticed.
Heat index / feels-like temperature: A measure combining air temperature and humidity to express how hot it actually feels to the human body, because humidity blocks cooling by sweat evaporation.
Wet-bulb temperature: A combined heat-humidity measure. Sustained wet-bulb temperatures near 35°C exceed the human body's ability to cool itself even at rest in shade — a threshold approached during the most extreme humid heatwaves.

Alerts & Monitoring

Watch vs Warning: A WATCH means conditions are favourable for a hazard — be prepared. A WARNING means the hazard is occurring or imminent — take protective action now. This distinction is used for tornadoes, hurricanes, floods and tsunamis.
GDACS alert levels: The Global Disaster Alert and Coordination System (EC/UN) grades events Green (international aid unlikely), Orange (potential regional impact) or Red (severe impact likely requiring international response), based on hazard size plus exposed population.
Earthquake early warning: Systems that detect an earthquake's fast P-waves and broadcast alerts seconds before damaging S-waves arrive. They provide seconds to tens of seconds of notice — not prediction.
EM-DAT: The International Disaster Database maintained by CRED at UCLouvain, Belgium — the most widely used global record of disaster occurrences and impacts since 1900.