Weather Glossary

Tornado

A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.

Supercell

A supercell is a thunderstorm with a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm: they produce nearly all significant tornadoes in the United States and most hail larger than golf balls, along with damaging winds and flash flooding.

Mesocyclone

A rotating vortex of air within a supercell thunderstorm, typically 2-6 miles in diameter. Mesocyclones are detected by Doppler radar and indicate the potential for tornado development.

Derecho

A derecho is a widespread, long-lived windstorm produced by a fast-moving line or cluster of severe thunderstorms. Under the criteria the Storm Prediction Center uses, the swath of wind damage must be at least 250 miles (about 400 km) long, with gusts of at least 58 mph along its length and several well-separated gusts of 75 mph or greater.

Microburst

A localized column of sinking air that produces damaging divergent winds at or near the surface. Microbursts can produce winds exceeding 100 mph in an area less than 2.5 miles across and are particularly hazardous to aircraft.

Downburst

A strong downdraft that induces an outward burst of damaging winds at or near the surface. Downbursts can be either microbursts (less than 2.5 miles) or macrobursts (larger than 2.5 miles) and can cause damage similar to a tornado.

Wall Cloud

An isolated lowering of a rain-free cloud base, typically found on the south or southwest side of a supercell. Wall clouds often rotate and may produce a funnel cloud or tornado. They usually precede tornado formation by 10-20 minutes.

Funnel Cloud

A rotating cone-shaped cloud extending downward from the base of a thunderstorm but not reaching the ground. When a funnel cloud touches the ground, it becomes a tornado.

Enhanced Fujita Scale

Also called: EF scale, EF rating

The Enhanced Fujita (EF) scale rates tornadoes from EF0 to EF5 based on the damage they cause. Surveyors compare the damage to 28 kinds of buildings, structures, and trees against expected levels of damage, then estimate the 3-second wind gust responsible. EF0 means estimated gusts of 65 to 85 mph, and EF5 means over 200 mph.

Flash Flood

A flash flood is a rapid and extreme flow of high water into a normally dry area, or a rapid rise in a stream or creek above flood level, that begins within about six hours of its cause. The usual cause is intense rainfall, but a dam failure or an ice jam can also trigger one.

Squall Line

Also called: quasi-linear convective system, QLCS

A squall line is a line of active thunderstorms, either continuous or with breaks, together with the rain areas the storms produce. Squall lines can extend for hundreds of miles and last for many hours. Their main hazard is straight-line wind damage, though tornadoes sometimes form along the leading edge.

Shelf Cloud

Also called: arcus cloud

A shelf cloud is a low, horizontal, wedge-shaped cloud attached to the base of a thunderstorm along its gust front, the leading edge of the storm's rain-cooled outflow. Rising cloud motion is often visible on its leading edge, while the underside looks turbulent and wind-torn. It means gusty winds are about to arrive.

Freezing Rain

Rain that falls as liquid but freezes on contact with surfaces at or below 32°F (0°C), creating a coating of ice called glaze. Freezing rain is extremely dangerous as it can make roads impassable and cause widespread power outages from ice-coated power lines.

Sleet

Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.

Hail

Balls or irregular lumps of ice produced in severe thunderstorms. Hailstones form when updrafts carry raindrops into extremely cold areas of the atmosphere where they freeze. Repeated cycles through the updraft can create hailstones larger than softballs.

Graupel

Soft, small pellets of ice formed when supercooled water droplets freeze onto falling snowflakes. Also called snow pellets, graupel is crumbly and opaque, unlike the hard, clear ice of hail. It often occurs in spring and fall storms.

Virga

Precipitation that falls from clouds but evaporates before reaching the ground. Virga appears as wispy streaks extending from the base of clouds and is common in dry or desert climates where the air below clouds has low humidity.

Snow Squall

An intense, short-lived burst of heavy snow accompanied by strong winds and often causing a sudden drop in visibility to near zero. Snow squalls can create extremely dangerous driving conditions despite lasting only 30-60 minutes.

Drizzle

Very light precipitation consisting of tiny water droplets less than 0.5mm in diameter that appear to float in the air. Drizzle typically falls from stratus clouds and reduces visibility more than light rain of the same intensity.

Thundersnow

A rare phenomenon where thunder and lightning occur during a snowstorm. Thundersnow typically happens in intense winter storms and is often associated with heavy snowfall rates of 2-4 inches per hour.

Atmospheric River

Also called: AR

An atmospheric river is a long, narrow corridor of concentrated water vapor moving through the lower atmosphere, typically 250 to 375 miles wide and often more than 1,000 miles long. When one comes ashore and is lifted over mountains, it produces prolonged heavy rain and snow. Atmospheric rivers are rated from AR 1 (weak) to AR 5 (exceptional) by strength and duration.

Pineapple Express

A strong atmospheric river that carries moisture from the tropical Pacific near Hawaii to the West Coast of North America. Because the air is warm, a Pineapple Express raises mountain snow levels, so more precipitation falls as rain and can melt existing snowpack, increasing the flood risk.

Barometric Pressure

The weight of the atmosphere pressing down on a given point, measured in inches of mercury (inHg) or millibars (mb). Rising pressure typically indicates improving weather, while falling pressure suggests approaching storms or precipitation.

Dew Point

The dew point is the temperature to which air must be cooled to become saturated, so that water vapor begins to condense into dew or fog. The higher the dew point, the more moisture is in the air. In summer, a dew point of 55°F or lower feels dry and comfortable, and 65°F or higher feels muggy.

Humidity

The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.

Relative Humidity

The ratio of the current absolute humidity to the maximum possible humidity at the current temperature, expressed as a percentage. 100% relative humidity means the air is fully saturated. Relative humidity changes with temperature even if moisture content stays the same.

Temperature Inversion

A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.

Wind Shear

A change in wind speed or direction over a short distance, either horizontally or vertically. Strong wind shear is essential for supercell and tornado development but is also a significant hazard to aircraft during takeoff and landing.

Heat Index

The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.

Wind Chill

Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.

Cold Front

A cold front is the boundary where a cooler, denser air mass is advancing and replacing warmer air. The cold air wedges under the warm air and forces it upward, which often produces a band of showers or thunderstorms, followed by a wind shift and cooler, drier air.

UV Index

Also called: ultraviolet index, UVI

The UV index is a forecast of the intensity of skin-damaging ultraviolet radiation expected to reach the ground at solar noon, when the sun is highest. It is 0 at night and rises to 11 or higher, and the higher the number, the faster unprotected skin and eyes are harmed. Sun protection is recommended at 3 and above.

Wet-Bulb Temperature

Also called: wet bulb

Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it. It is always at or below the air temperature, and the drier the air, the larger the gap between the two. Forecasters use it to judge heat stress and how much falling rain or snow can cool the air.

NEXRAD

Next-Generation Radar, a network of 159 high-resolution Doppler weather radars operated by the National Weather Service. NEXRAD detects precipitation, wind patterns, and severe weather signatures up to 250 miles from each station.

MRMS

Multi-Radar Multi-Sensor, an advanced system that combines data from multiple radars, satellites, and surface observations to create a seamless, high-resolution national weather picture updated every 2 minutes.

Reflectivity

A measure of how much radar energy is reflected back by precipitation, measured in dBZ (decibels of Z). Higher values indicate heavier precipitation or larger particles. Values above 50 dBZ typically indicate heavy rain or hail.

Velocity

Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.

Echo Tops

The maximum height at which radar detects precipitation in a storm, indicating storm intensity and updraft strength. Higher echo tops generally correlate with stronger storms. Tops exceeding 40,000 feet suggest severe weather potential.

Hook Echo

A distinctive hook-shaped radar signature on the right rear of a supercell thunderstorm, indicating rotation and high tornado probability. Hook echoes form as precipitation wraps around the mesocyclone.

Bow Echo

A bow or crescent-shaped radar signature indicating strong straight-line winds. Bow echoes form when the center of a squall line accelerates ahead of its ends, often producing damaging winds exceeding 75 mph.

Debris Ball

A radar signature showing a concentrated area of debris lofted by a tornado. Debris balls appear as a circular area of high reflectivity collocated with rotation and confirm a tornado is on the ground causing damage.

Doppler Radar

Doppler radar is weather radar that measures the motion of precipitation as well as its location and intensity. It sends out pulses of radio waves and detects the shift in phase of the signal that returns, which shows whether targets are moving toward or away from the radar and how fast. That reveals wind patterns and rotation inside storms.

dBZ

Also called: decibels of Z, decibels relative to Z

dBZ, short for decibels of Z, is the unit of radar reflectivity: a logarithmic measure of how much energy precipitation returns to the radar. The scale runs from about -35 to +85 dBZ. As a guide, 20 to 40 dBZ is light precipitation, 40 to 50 is moderate, and 50 to 65 is heavy rain or some hail.

Correlation Coefficient

Also called: CC

Correlation coefficient (CC) is a dual-polarization radar product that measures how similar the horizontal and vertical radar pulses are after they hit their targets, on a scale from 0 to 1. Uniform rain or snow gives values above 0.97. Mixed precipitation and hail give lower values, and tornado debris typically falls below 0.80.

Skew-T

Also called: Skew-T Log-P diagram, sounding

A Skew-T Log-P diagram is the chart meteorologists use to plot a weather balloon sounding: temperature, dew point, and wind from the ground to the upper atmosphere. Its temperature lines are tilted 45 degrees, which gives the chart its name. A hodograph, often shown beside it, plots how wind speed and direction change with height.

SPC Outlook

Severe weather forecasts issued by the Storm Prediction Center categorizing risk levels from Marginal (1) to High (5). Outlooks cover tornadoes, damaging winds, and large hail for 1-3 days ahead and highlight where organized severe weather is most likely.

Watch vs. Warning

A watch means the risk of hazardous weather has risen but its timing, location, or occurrence is still uncertain: be prepared. A warning means hazardous weather is occurring, imminent, or likely and threatens life or property: take action now. An advisory covers events that are occurring or likely but less serious than a warning.

CAPE

Also called: Convective Available Potential Energy

CAPE (Convective Available Potential Energy) is the amount of energy a rising parcel of air can draw from the atmosphere, measured in joules per kilogram (J/kg). The higher the CAPE, the faster a storm's updraft can rise, and the stronger thunderstorms can become once something sets them off.

Lifted Index

A stability index comparing the temperature of a rising air parcel to the environment at 500mb (about 18,000 feet). Negative values indicate instability. Values below -6 suggest strong instability and severe thunderstorm potential.

NAM

North American Mesoscale Model, a regional weather model covering North America at 3-12 km resolution and NCEP's short-range workhorse for mesoscale features like fronts, lake-effect snow, and sea breezes. Its successor is the Rapid Refresh Forecast System (RRFS) with its ensemble REFS, and the same changeover ends SREF, HREF, HiresW, and NAM MOS. The National Weather Service notice of October 2, 2026 set the changeover date as November 3, 2026, moved from October 14.

GFS

Global Forecast System, the primary American global weather model run by NOAA. The GFS provides forecasts out to 16 days and is widely used for medium-range forecasting. It runs four times daily with global coverage.

HRRR

High-Resolution Rapid Refresh, a real-time 3km resolution weather model updated hourly. The HRRR excels at short-range (0-18 hour) forecasts of convection, precipitation, and aviation weather across the contiguous United States.

Ensemble Forecast

A collection of multiple model runs with slightly different initial conditions, showing the range of possible outcomes. Ensemble forecasts help quantify uncertainty and are especially valuable for medium-range predictions.

Probability of Precipitation

Also called: PoP, chance of rain

Probability of precipitation (PoP) is the chance that a given point receives measurable precipitation, at least 0.01 inch, during the forecast period. A 40% chance of rain means a 4 in 10 chance of at least 0.01 inch at your location. It does not mean rain over 40% of the area or for 40% of the time.

ECMWF

Also called: European model, Euro, IFS

ECMWF is the European Centre for Medium-Range Weather Forecasts, an intergovernmental organization supported by 35 states, and the name commonly given to its global weather model, often called the European model or the Euro. Its physics-based Integrated Forecasting System (IFS) runs on a global grid of about 9 km and forecasts up to 15 days ahead.

Zulu Time

Also called: UTC, Z time, Coordinated Universal Time

Zulu time, written with a Z, is the name used in aviation and meteorology for Coordinated Universal Time (UTC), the time at the Greenwich prime meridian. Weather observations, model runs, and aviation reports all use it so that a time means the same thing everywhere. 12Z is 8 in the morning Eastern Daylight Time and 7 in the morning Eastern Standard Time.

METAR

Meteorological Aerodrome Report, a standardized format for reporting current weather conditions at airports. METARs are issued hourly (or more frequently during significant changes) and include wind, visibility, clouds, temperature, and pressure.

TAF

Terminal Aerodrome Forecast, a weather forecast for the area within 5 miles of an airport. TAFs are issued every 6 hours and cover a 24-30 hour period, providing expected winds, visibility, clouds, and precipitation.

Ceiling

The height above ground of the lowest broken or overcast cloud layer, or vertical visibility into an obscuration. Scattered clouds do not form a ceiling. A ceiling below 1,000 feet falls in the IFR or LIFR weather category; legal flight and approach requirements depend on airspace, visibility, regulations, equipment, and pilot qualifications.

Visibility

The horizontal distance at which objects can be seen and identified. U.S. aviation reports express prevailing visibility in statute miles. Below 3 statute miles falls in the IFR or LIFR weather category, but legal visual-flight minimums vary with airspace, altitude, and day or night under 14 CFR 91.155.

IFR

Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.

VFR

Visual Flight Rules allow flight using visual references when the applicable visibility and cloud-clearance minimums are met. Those minimums vary by airspace class, altitude, and day or night under 14 CFR 91.155. Separately, the VFR weather category means a ceiling above 3,000 feet and visibility greater than 5 statute miles.

MVFR

Marginal Visual Flight Rules, conditions between VFR and IFR with ceiling 1000-3000 feet and/or visibility 3-5 miles. MVFR indicates reduced but still visual conditions requiring extra caution.

SIGMET

Significant Meteorological Information, an advisory for hazardous weather affecting all aircraft including severe turbulence, icing, volcanic ash, and thunderstorms. SIGMETs cover large areas and last up to 4 hours.

AIRMET

Airmen's Meteorological Information, an advisory for weather hazardous primarily to light aircraft including moderate turbulence, icing, and low visibility. AIRMETs are less severe than SIGMETs.

Density Altitude

The altitude at which the current air density would be found in a standard atmosphere. High temperatures and humidity increase density altitude, reducing aircraft performance. Critical for takeoff and landing calculations at high-elevation airports.

LIFR

Also called: Low Instrument Flight Rules, Low IFR

LIFR, Low Instrument Flight Rules, is the lowest of the four aviation weather flight categories. It means a ceiling below 500 feet and/or visibility of less than 1 statute mile. Whichever of the two is lower sets the category. The categories above it, in order, are IFR, MVFR, and VFR.

PIREP

Also called: pilot report, pilot weather report

A PIREP, or pilot report, is a report of the actual weather an aircraft encounters in flight, made by the pilot or a crew member. PIREPs describe conditions such as turbulence, icing, and cloud layers at a stated location and altitude, which fills the gaps between ground stations with direct observations from the air.

Hurricane

A tropical cyclone with maximum one-minute sustained surface winds of 74 mph or greater in the Atlantic or eastern and central North Pacific. Hurricanes are categorized 1-5 on the Saffir-Simpson Hurricane Wind Scale, with Category 5 beginning at 157 mph.

Typhoon

A tropical cyclone with sustained winds of 74 mph or greater in the Western Pacific. Typhoons are meteorologically identical to hurricanes; the different name reflects only their geographic location.

Cyclone

A rotating low-pressure weather system. Tropical cyclone terminology and intensity categories vary by region: in Australia, a tropical cyclone has maximum sustained winds of at least 34 knots (63 km/h, about 39 mph), averaged over ten minutes. The term does not universally require hurricane-force winds of 74 mph.

Eye Wall

The ring of intense thunderstorms surrounding a hurricane's eye, containing the storm's strongest winds and heaviest rainfall. Eye wall replacement cycles can temporarily weaken a hurricane before it potentially re-intensifies.

Storm Surge

Storm surge is an abnormal rise in sea level during a hurricane or other intense storm, measured as the height of the water above the normal predicted tide. It is produced mainly by the storm's winds pushing water toward the shore. Storm surge is the leading cause of deaths from hurricanes.

Tropical Depression

An organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph or less. Tropical depressions can strengthen into tropical storms and eventually hurricanes.

Tropical Storm

A tropical cyclone with maximum sustained winds between 39-73 mph. At this stage, the storm receives a name. Tropical storms can produce dangerous flooding, winds, and tornadoes even without reaching hurricane strength.

Invest

A weather system designated for investigation by the National Hurricane Center. An Invest designation triggers enhanced data collection and model runs but does not indicate expected development. Many Invests never become tropical cyclones.

Saffir-Simpson Scale

Also called: Saffir-Simpson Hurricane Wind Scale, hurricane categories

The Saffir-Simpson Hurricane Wind Scale rates hurricanes from Category 1 to Category 5 by maximum sustained wind speed. Category 1 starts at 74 mph and Category 5 at 157 mph, and Category 3 and higher are called major hurricanes. The scale measures wind only: it does not account for storm surge, rainfall flooding, or tornadoes.

Post-Tropical Cyclone

Also called: remnant low, extratropical cyclone

A post-tropical cyclone is a former tropical cyclone that no longer has enough tropical characteristics to be classified as one. It can still bring heavy rain and strong winds. The two kinds are extratropical cyclones, which draw their energy from temperature contrasts between air masses, and remnant lows, which have winds below 34 knots (39 mph).

Frost

Frost is a thin layer of ice crystals that forms on surfaces when they cool to 32°F (0°C) or below and water vapor in the air freezes onto them. It is the surface that has to reach freezing, not the air: frost can form while the official air temperature is still in the mid 30s.

Black Ice

A thin, transparent layer of ice on road surfaces that is nearly invisible, making it extremely dangerous for drivers. Black ice forms when temperatures drop rapidly after rain or from refreezing meltwater, often at night or in shaded areas.

Lake Effect Snow

Intense, localized snowfall produced when cold air moves across relatively warm lake water, picking up moisture and heat. Lake effect snow can produce snowfall rates of 2-4 inches per hour and create dramatic differences in accumulation over short distances.

Alberta Clipper

A fast-moving low-pressure system that originates near Alberta, Canada, and tracks southeast across the northern United States. Clippers typically bring light snow (1-4 inches) followed by bitter cold and gusty winds.

Nor'easter

A storm along the East Coast with winds blowing from the northeast. Nor'easters can bring heavy snow, rain, and coastal flooding, especially from Virginia to New England. The most powerful nor'easters rival hurricanes in intensity.

Freezing Fog

Fog that forms when air temperature is below 32°F (0°C), depositing ice crystals on exposed surfaces. Freezing fog creates treacherous driving conditions and can coat everything with a layer of rime ice.

Ice Storm

A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.

Blizzard

A blizzard is a storm with sustained winds or frequent gusts of 35 mph or more and falling or blowing snow that frequently reduces visibility to below a quarter mile, lasting 3 hours or longer. The National Weather Service definition has no snowfall or temperature requirement: wind and visibility make a blizzard.

Bomb Cyclone

Also called: bombogenesis, explosive cyclogenesis

A bomb cyclone is a midlatitude storm that intensifies rapidly through a process called bombogenesis: its central pressure falls by at least 24 millibars in 24 hours, measured at 60° latitude. The threshold shrinks toward the equator, to about 17.8 millibars at the latitude of New York City.

Rip Current

A rip current is a strong, narrow channel of water flowing away from the beach through the surf zone. Rip currents can form on any beach with breaking waves, including the Great Lakes, and can reach 8 feet per second, faster than an Olympic swimmer. They carry swimmers away from shore, not under the water.

Small Craft Advisory

Also called: SCA

A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.

King Tide

Also called: perigean spring tide

King tide is a popular, non-scientific name for an exceptionally high tide. King tides happen when the higher tides around a new or full moon coincide with the Moon's closest approach to Earth, called perigee. They can flood low-lying coastal areas on a clear day, and they raise the water level any storm surge is added to.

AQI

Also called: Air Quality Index

The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.

PM2.5

Also called: fine particulate matter, fine particles

PM2.5 is fine particulate matter: airborne particles 2.5 micrometers or smaller in diameter. An average human hair is about 30 times wider than the largest of them. The particles are small enough to reach deep into the lungs, and some enter the bloodstream, which makes PM2.5 the particle pollution with the greatest health risk.

Red Flag Warning

A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.

El Niño and La Niña

Also called: ENSO, El Niño-Southern Oscillation

El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.

Jet Stream

A jet stream is a relatively narrow band of strong wind in the upper atmosphere, typically around 30,000 feet, that blows from west to east. The polar jet sits between 50° and 60° latitude and the subtropical jet near 30°. Jet streams follow the boundaries between cold and warm air and steer the storm systems below them.

Polar Vortex

The polar vortex is a large area of low pressure and cold air surrounding each of Earth's poles. It always exists, weakening in summer and strengthening in winter. During Northern Hemisphere winter it often expands, sending Arctic air south with the jet stream and producing cold outbreaks across the United States.

Kp Index

Also called: planetary K-index, K-index

The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.

Magnitude

A single number estimating the size of an earthquake at its source, derived from seismograms. Older scales read it from the peak amplitude of the recorded waves; the modern standard, moment magnitude, is computed from the physical size of the fault rupture instead. The scale is logarithmic: each whole number represents about 10 times more ground motion and roughly 32 times more energy released. Magnitude describes the earthquake itself, not how strongly it was felt at any particular place.

Moment Magnitude

The modern standard magnitude scale (Mw), based on seismic moment: the rupture area multiplied by the average slip and the rigidity of the rock. Unlike older scales it does not saturate, so it stays accurate for the largest earthquakes. USGS reports it under labels such as Mww, derived from W-phase moment tensor solutions.

Richter Scale

The original local magnitude scale (ML) developed by Charles Richter in 1935 for Southern California. It saturates above roughly magnitude 7, meaning it underestimates the largest earthquakes, so seismologists replaced it with moment magnitude for significant events. News coverage still refers to the Richter scale out of habit even when the figure quoted is actually moment magnitude.

Epicenter

The point on Earth's surface directly above where a fault rupture begins. It is a single map location, not the whole earthquake: a large event can break hundreds of kilometers of fault, so severe shaking often extends far beyond the epicenter.

Hypocenter

The actual point underground where fault rupture begins, also called the focus. It is described by latitude, longitude, and depth. The epicenter is simply the hypocenter projected straight up to the surface.

Focal Depth

How far below the surface an earthquake began, reported in kilometers. Shallow earthquakes (roughly 0 to 70 km) shake the surface hardest because the energy travels a shorter distance; very deep earthquakes can be felt across huge areas but often cause less damage. When depth cannot be resolved from the available data, USGS may fix it at a default value such as 10 km.

Fault

A fracture in Earth's crust where blocks of rock have moved past one another. Earthquakes occur when accumulated stress overcomes friction on a fault and the rock slips suddenly. The main types are strike-slip (horizontal motion, as on the San Andreas), normal (pulling apart), and reverse or thrust (compression).

Aftershock

A smaller earthquake following a mainshock in the same area, caused by the crust adjusting to redistributed stress. Aftershocks are most frequent in the first hours and days and taper off over time, but can continue for months or years after a large event. If one turns out to be larger than the original earthquake, it is reclassified as the mainshock.

Foreshock

An earthquake that precedes a larger event on or near the same fault. Foreshocks can only be identified after the fact, which is one reason earthquakes cannot be reliably predicted: at the time it happens, a foreshock looks like any other small earthquake.

Earthquake Swarm

A cluster of earthquakes in a small area over days to months with no single dominant mainshock. Swarms are common in volcanic and geothermal regions, where moving magma or underground fluids change stress in the surrounding crust.

P-Wave

The primary or compressional wave, the fastest seismic wave and the first to reach any recording station. P-waves push and pull rock along their direction of travel and can pass through both solids and liquids. Their head start over more damaging waves is what makes earthquake early warning physically possible.

S-Wave

The secondary or shear wave, which arrives after the P-wave and moves rock perpendicular to its direction of travel. S-waves shake far more strongly than P-waves and cause much of the damage in an earthquake. They cannot travel through liquids, which is how scientists established that Earth's outer core is molten.

Seismometer

An instrument that detects and records ground motion; the recording it produces is called a seismogram. Networks of seismometers locate an earthquake by comparing wave arrival times across many stations, which is why reported location, depth, and magnitude get more accurate as additional stations report in.

Modified Mercalli Intensity

A scale from I to XII describing how strongly shaking was felt and what damage occurred at a specific location, rather than the size of the earthquake at its source. One earthquake has a single magnitude but many intensity values, which decrease with distance and vary sharply with local soil conditions.

ShakeMap

A USGS product that maps estimated ground shaking and intensity across an affected region within minutes of an earthquake, combining instrument recordings, public felt reports, and models. Emergency managers use it to identify where damage is most likely before reports arrive from the ground.

Liquefaction

A process in which water-saturated, loosely packed soil temporarily behaves like a liquid during strong shaking and loses its ability to bear weight. It can tilt or sink buildings, float buried tanks and pipes upward, and cause the ground to crack and spread sideways. Reclaimed land, river deltas, and sandy fill are especially vulnerable.

Subduction Zone

A plate boundary where one tectonic plate is forced beneath another. Subduction zones produce the largest earthquakes on Earth, including every recorded magnitude 9 event, along with deep earthquakes, chains of volcanoes, and most destructive tsunamis.

Ring of Fire

The horseshoe-shaped belt of subduction zones, faults, and volcanic arcs surrounding the Pacific Ocean, running from New Zealand up through Asia, across Alaska, and down the Americas. About 90 percent of the world's earthquakes occur along it.

Tsunami Warning

The highest alert level issued by a tsunami warning center, meaning a dangerous tsunami is expected or already underway and people in the threatened zone should move inland or to high ground immediately. The lower levels are an advisory (dangerous currents near the water), a watch (a distant event may pose a threat, stay alert), and an information statement (no threat expected).

Earthquake Early Warning

A system that detects the fast P-wave near an earthquake's origin and issues an alert before the slower, more damaging shaking arrives, providing seconds to tens of seconds of notice. It is detection, not prediction: the earthquake has already begun. Public systems operate in regions including the U.S. West Coast (ShakeAlert), Japan, and Mexico.