Weather Glossary
Comprehensive guide to weather terminology. Tap a category to jump to that section, or click any term to learn more.
Severe Weather
Terms related to dangerous weather phenomena including tornadoes, thunderstorms, and damaging winds.
Tornado
A violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can produce winds exceeding 300 mph and cause catastrophic damage along their path. They are rated on the Enhanced Fujita (EF) scale from EF0 to EF5.
Supercell
A severe thunderstorm characterized by a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm and can produce tornadoes, large hail, 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 widespread, long-lived windstorm associated with a fast-moving band of severe thunderstorms. To qualify as a derecho, the wind damage swath must extend more than 250 miles with wind gusts of at least 58 mph along most of its length.
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.
Precipitation
Types of water falling from clouds, including rain, snow, and mixed precipitation.
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
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 an atmospheric river comes ashore and is lifted over mountains, it produces prolonged heavy rain and snow. Atmospheric rivers supply 30 to 50% of the West Coast's annual precipitation and 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.
Atmospheric Conditions
Measurements and phenomena describing the state of the atmosphere.
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 temperature at which air becomes saturated and water vapor begins to condense into liquid. A higher dew point indicates more moisture in the air. Dew points above 65°F feel humid, while above 70°F is considered oppressive.
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
A measure of how hot it feels when humidity is factored in with the actual air temperature. High humidity prevents sweat from evaporating efficiently, making the body feel hotter. Heat illness can occur at lower values too, depending on sun, exertion, exposure time, and individual health.
Wind Chill
A measure of how cold it feels on exposed skin when wind is factored in with the actual air temperature. Wind accelerates heat loss from the body, and the NWS formula uses air temperature and wind speed: at 0°F with a 15 mph wind, the wind chill is -19°F. For that 0°F and 15 mph combination, the NWS chart indicates frostbite can occur on exposed skin in about 30 minutes. Check the chart for other combinations.
Radar & Observation
Weather radar systems, products, and observational terminology.
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.
Forecasting & Models
Weather prediction systems, model outputs, and forecast terminology.
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 conditions are favorable for hazardous weather to develop: be prepared. A Warning means hazardous weather is imminent or occurring: take action immediately. Warnings are more urgent than watches.
CAPE
Convective Available Potential Energy, a measure of atmospheric instability and thunderstorm potential. CAPE values above 1000 J/kg indicate instability, 2500 to 3500 J/kg is very unstable, and extreme instability starts above roughly 3500 to 4000 J/kg.
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. NCEP retires the NAM at 12 UTC on October 14, 2026, along with SREF, HREF, HiresW, and NAM MOS; the Rapid Refresh Forecast System (RRFS) and its ensemble REFS take over that day.
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.
Aviation Weather
Specialized weather terminology used in aviation forecasting and operations.
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.
Tropical Weather
Terminology related to hurricanes, tropical storms, and coastal weather hazards.
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
An abnormal rise of seawater generated by a storm's winds pushing water toward shore. Storm surge is the deadliest hurricane hazard, capable of raising water levels 20+ feet and flooding coastal areas miles inland.
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.
Winter Weather
Cold weather phenomena and winter storm terminology.
Frost
Ice crystals that form on surfaces when the temperature drops to 32°F (0°C) or below and moisture in the air freezes. Frost can damage crops and plants and makes surfaces slippery. A frost advisory warns of light frost; a freeze warning indicates sustained freezing temperatures.
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.
Earthquakes & Seismology
Terms for reading earthquake reports: magnitude scales, depth, faults, seismic waves, shaking intensity, and tsunami alert levels.
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.
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