A
Advection
The horizontal transport of an atmospheric property — usually heat or moisture — by wind.
Aerosol
Tiny solid or liquid particles suspended in the atmosphere. Includes dust, smoke, sea salt, sulfates.
Air mass
A large body of air with fairly uniform temperature and humidity throughout.
Albedo
The fraction of incoming solar radiation reflected by a surface. Fresh snow ≈ 0.9; asphalt ≈ 0.05.
Altimeter
An instrument that measures altitude, usually via barometric pressure.
Altocumulus
Mid-level puffy cloud clumps, often in rows. Sometimes called 'mackerel sky'.
Learn more →Altostratus
Mid-level grey or blue-grey layered cloud that dims the sun without producing sharp shadows.
Anticyclone
A high-pressure system with clockwise flow in the Northern Hemisphere. Generally fair weather.
Anvil
The flat, spreading top of a mature thunderstorm where the updraft hits the tropopause.
Aphelion
The point in Earth's orbit farthest from the Sun. Occurs in early July.
Atmospheric pressure
The weight of the column of air above a given point. Measured in millibars or inches of mercury.
Aurora
Colorful light in the polar sky caused by charged particles interacting with the upper atmosphere.
B
Backing wind
Wind that shifts counterclockwise with time (e.g. south → southeast → east).
Ball lightning
A rare, unexplained glowing sphere sometimes seen during thunderstorms.
Barometer
An instrument that measures atmospheric pressure.
Beaufort scale
A wind-speed scale from 0 (calm) to 12 (hurricane force).
Learn more →Beaver's tail
A long horizontal cloud band on the northeast side of a supercell marking the inflow-outflow boundary.
Learn more →Bergeron process
The process by which ice crystals grow at the expense of supercooled water droplets in a cloud.
Blizzard
A storm with winds of at least 35 mph and visibility under a quarter mile for at least three hours.
Blowing snow
Snow lifted from the surface by wind to a height of 2 meters or more.
Blue Norther
A rapid drop in temperature in Texas caused by a cold front from the north; often 30°F+ in an hour.
Bow echo
A bow-shaped line of thunderstorms on radar. Often produces damaging straight-line winds.
BWER
Bounded Weak Echo Region — a signature of a very strong updraft on radar; suggests supercell.
C
Cap
A layer of warm air aloft that suppresses convection. Also called CIN.
CAPE
Convective Available Potential Energy — the 'fuel' available to storm updrafts. Higher = stronger storms.
Learn more →Ceiling
The height of the lowest layer of clouds that covers more than half the sky.
Cell
A single convective updraft or storm cluster.
Cirrus
High, thin, wispy ice-crystal cloud above 20,000 ft.
Learn more →Climate
The long-term average of weather conditions in a region (typically 30 years).
Cold front
The leading edge of a colder air mass replacing warmer air. Often triggers storms.
Condensation
The transition of water vapor into liquid water.
Conduction
Heat transfer by direct contact between molecules.
Convection
Heat transfer by fluid motion, such as rising warm air.
Convergence
The horizontal flow of air into a region — creates rising motion.
Coriolis effect
The apparent deflection of moving objects (including air) by Earth's rotation.
Corona
A ring of colored light seen around the sun or moon through thin water-droplet clouds.
Cumulonimbus
A towering vertical cloud that produces thunderstorms, hail and tornadoes.
Learn more →Cumulus
Puffy, fair-weather cloud with a flat base and cauliflower top.
Learn more →Cyclone
A rotating low-pressure system. In the Indian Ocean and South Pacific also refers to hurricanes.
D
Debris signature
A radar signature indicating lofted tornado debris; a strong tornado confirmation.
Learn more →Derecho
A widespread, fast-moving line of severe wind storms travelling hundreds of miles.
Learn more →Dew
Water droplets that condense on surfaces cooler than the dew point of the air.
Dew point
The temperature to which air must be cooled to become saturated.
Diffluence
The spreading apart of streamlines in the atmosphere; often signals rising motion aloft.
Divergence
The horizontal outflow of air from a region — creates sinking motion or supports rising below.
Doppler radar
A radar that measures the velocity of precipitation particles by frequency shift.
Downburst
A strong downward burst of air from a thunderstorm. Straight-line winds can top 100 mph.
Downdraft
A localized column of sinking air within a thunderstorm.
Dryline
A boundary between moist and dry air masses, especially in the Southern Plains. Common tornado trigger.
Dust devil
A small, short-lived, non-thunderstorm whirlwind over dry ground.
Learn more →E
EF Scale
Enhanced Fujita Scale — the tornado damage rating scale (EF0–EF5).
Learn more →EHI
Energy-Helicity Index — a composite parameter of CAPE and 0-3 km storm-relative helicity.
El Niño
A periodic warming of eastern equatorial Pacific waters that alters global weather patterns.
ENSO
El Niño–Southern Oscillation — the coupled ocean-atmosphere phenomenon in the tropical Pacific.
Equinox
One of two days per year when day and night are approximately equal in length.
Evaporation
The transition of liquid water into water vapor.
Eye
The relatively calm center of a hurricane, surrounded by the eyewall.
Eyewall
The ring of intense thunderstorms surrounding the eye of a hurricane; strongest winds.
F
Fetch
The distance over which wind blows across water. Longer fetch = bigger waves.
Flash flood
Rapid flooding of low-lying areas within hours of intense rainfall.
Fog
A cloud that touches the ground, reducing visibility below 1 km.
Freezing rain
Rain that falls as liquid but freezes on contact with surfaces, coating them in ice.
Front
The boundary between two air masses of different temperature or moisture.
Frost
Ice crystals that form when water vapor deposits directly onto a cold surface.
Fujita scale
The original tornado damage scale (F0–F5) introduced in 1971.
Funnel cloud
A rotating cone-shaped cloud extending from a cumulonimbus base but not reaching the ground.
G
Gale
A sustained wind of 34-40 knots (39-46 mph).
Geostrophic wind
The theoretical wind that flows parallel to isobars, balanced between pressure gradient and Coriolis.
Graupel
Soft, granular snow pellets — snowflakes coated with rime.
Green sky
A greenish tint sometimes seen in severe thunderstorms; caused by light scattering through hail.
Greenhouse effect
The warming of Earth's surface caused by atmospheric gases trapping outgoing longwave radiation.
Gust front
The leading edge of a thunderstorm's cold outflow, often marked by a shelf cloud.
Gustnado
A short-lived swirl on the leading edge of a thunderstorm outflow; not a true tornado.
H
Haboob
A massive dust storm generated by a thunderstorm outflow, especially in deserts.
Hail
Precipitation of solid ice; forms in strong updrafts of thunderstorms.
Halo
A ring of light around the sun or moon, caused by ice crystals in cirrostratus.
Haze
A suspension of fine particles that reduces atmospheric visibility.
Heat lightning
Distant lightning from a thunderstorm too far to hear the thunder; not a separate type.
Helicity
A measure of the potential for rotation in the atmosphere.
High pressure
An area of higher-than-average atmospheric pressure; usually associated with fair weather.
Hodograph
A graph plotting wind vectors at multiple heights to visualize wind shear.
Hook echo
A hook-shaped radar signature indicating a tornado-producing supercell.
Learn more →HP supercell
High-precipitation supercell — a supercell where the tornado is buried inside rain.
Humidity
The amount of water vapor in the air. Relative humidity is a percentage of saturation.
Hurricane
A tropical cyclone with sustained winds ≥74 mph in the Atlantic or Eastern Pacific.
Hydrologic cycle
The continuous movement of water between the atmosphere, oceans, land and living things.
Hygrometer
An instrument that measures humidity.
M
Mackerel sky
A sky covered with cirrocumulus that looks like fish scales.
Mammatus
Pouch-like clouds hanging beneath the anvil of a thunderstorm.
Learn more →Mercury
A silvery liquid metal used in older barometers and thermometers.
Mesocyclone
A rotating updraft within a supercell — the parent circulation of most tornadoes.
Learn more →Mesoscale
Atmospheric phenomena on scales of a few km to a few hundred km.
Microburst
A small, intense downburst under 2.5 miles across; dangerous for aviation.
Millibar
A unit of atmospheric pressure. Standard sea-level pressure is 1013 mb.
Mist
Very fine water droplets suspended in air, reducing visibility but not as much as fog.
Monsoon
A seasonal wind pattern that reverses direction between summer and winter.
Multi-vortex tornado
A tornado with multiple sub-vortices rotating around the main center.
Learn more →Multicell
A thunderstorm cluster made up of multiple cells in different life stages.
R
Radiosonde
An instrument carried aloft by a weather balloon to measure atmospheric conditions.
Rain
Liquid precipitation with drops larger than 0.5 mm.
Rain shadow
The dry region on the leeward side of a mountain range.
Rainbow
A meteorological phenomenon where sunlight is refracted and reflected in water droplets.
Rapid intensification
An increase in tropical cyclone winds of ≥35 mph in 24 hours.
RFD
Rear-flank downdraft — a plume of dry sinking air behind a supercell's mesocyclone.
Ridge
An elongated area of high pressure.
Rime
White ice deposited on surfaces by supercooled fog or cloud droplets freezing on contact.
S
Saffir-Simpson scale
The hurricane wind-intensity scale (Category 1 through 5).
Satellite tornado
A small tornado orbiting a larger primary tornado.
Severe thunderstorm
A thunderstorm with hail ≥1 inch, winds ≥58 mph, or a tornado.
Shelf cloud
A low horizontal wedge-shaped cloud on the leading edge of a thunderstorm's outflow.
Learn more →Shower
A short-duration precipitation event from a convective cloud.
Sleet
Frozen precipitation that reaches the ground as ice pellets.
Snow
Frozen precipitation composed of ice crystals.
Snow squall
A brief, intense snow shower with strong winds; often produces whiteout conditions.
Solstice
The two days of the year with the longest or shortest daylight (June 21 / December 21).
Sounding
A vertical profile of atmospheric conditions taken by a radiosonde.
SPC
Storm Prediction Center — the NOAA arm that issues severe weather outlooks.
Sprite
A brief red flash of light high above thunderclouds, in the mesosphere.
Squall line
A long, narrow line of severe thunderstorms.
SRH
Storm-Relative Helicity — a measure of low-level rotation available for tornado formation.
Learn more →Storm surge
The abnormal rise of seawater during a hurricane, driven by wind and low pressure.
Stratocumulus
Low, lumpy grey clouds usually covering the whole sky; light drizzle possible.
Stratus
A low, uniform layer of grey cloud that often produces mist or drizzle.
Learn more →Sublimation
The direct transition of ice to water vapor without becoming liquid.
Subtropical
A tropical cyclone with characteristics of both tropical and extratropical systems.
Sun dog
A bright colored spot on either side of the sun caused by ice crystals; a solar parhelion.
Supercell
A thunderstorm with a persistent rotating updraft; produces most significant tornadoes.
Learn more →Supersaturation
The condition where relative humidity exceeds 100%.
Synoptic scale
The largest atmospheric scale, on the order of 1000+ km.
T
TDS
Tornado Debris Signature — the radar debris ball indicating an ongoing tornado.
Thermal
A rising column of warm air; how sailplanes and eagles gain altitude.
Thermometer
An instrument that measures temperature.
Thunder
The sound produced by rapidly expanding air heated by a lightning stroke.
Thundersnow
A thunderstorm that produces snow instead of rain.
Tornado
A violently rotating column of air in contact with both the ground and a cumulonimbus cloud.
Tornado Alley
The traditional region of highest tornado frequency from Texas through South Dakota.
Learn more →Tornado warning
Issued when a tornado has been sighted or indicated by radar. Take shelter immediately.
Tornado watch
Issued when conditions are favorable for tornadoes in and near the watch area.
Tropical cyclone
The generic name for a hurricane, typhoon or cyclone.
Tropical depression
A tropical cyclone with winds under 39 mph.
Tropical storm
A tropical cyclone with sustained winds 39-73 mph.
Tropopause
The boundary between the troposphere and stratosphere; where the anvil forms.
Troposphere
The lowest layer of Earth's atmosphere, where most weather happens (0-10 km).
Trough
An elongated area of low pressure; often triggers storms.
Tsunami
A large sea wave, usually caused by an earthquake; not meteorological.
Twister
Colloquial term for a tornado.
Typhoon
A tropical cyclone in the northwestern Pacific.
W
Wall cloud
A localized lowering of a rain-free cloud base; often precedes tornado formation.
Learn more →Warm front
The leading edge of a warmer air mass replacing colder air.
Waterspout
A tornado over water.
Weather balloon
A balloon carrying a radiosonde into the upper atmosphere.
Wedge tornado
A large, wide tornado, often wider than it is tall.
Wet-bulb temperature
The lowest temperature achievable by evaporative cooling.
Whiteout
A condition of near-zero visibility caused by heavy snow, blowing snow, or fog.
Wind chill
The perceived temperature accounting for the cooling effect of wind on exposed skin.
Wind shear
A change in wind speed or direction with height or horizontal distance.
WSR-88D
The current generation of NOAA's Doppler weather radar.