Weather & Aviation Glossary

Area Forecast Discussion

Forecaster's narrative explaining the weather situation

Written by NWS meteorologists four times a day, explaining their reasoning and confidence levels. Great for understanding the 'why' behind forecasts — a TAF says the wind shifts at 18Z, the discussion says it shifts because a cold front arrives and that the forecaster is unsure of the timing. PlaneWX reads the discussion for your airport's office and quotes it directly.

Above Ground Level

Height measured from the ground surface

Airmen's Meteorological Information

Weather advisory for conditions hazardous to small aircraft

Issued for IFR conditions, mountain obscuration, moderate turbulence, sustained surface winds ≥30 kts, and moderate icing.

Air Route Traffic Control Center

En route ATC facility managing high-altitude airspace

There are 21 ARTCCs (Centers) in the U.S., each controlling a large sector of airspace. They issue Center Weather Advisories (CWAs) for their region.

Aviation Weather Center

NOAA center for aviation weather products

Primary source for AIRMETs, SIGMETs, and other aviation-specific weather products.

Becoming

Conditions changing to new state during period

Broken

5/8 to 7/8 sky coverage (62-87% clouds)

Mist

Visibility 5/8 to 6 SM due to water droplets

Clear Air Turbulence

Turbulence outside of clouds, typically at upper altitudes

Caused by strong vertical wind shear, often near the jet stream or along frontal boundaries. Undetectable by radar — forecast models and PIREPs are the primary detection tools. Severity depends on wind shear magnitude; the Richardson number indicates whether conditions support turbulence development.

Ceiling And Visibility Unlimited

Perfect flying weather

Ceiling above 10,000 ft and visibility greater than 10 SM. Ideal VFR conditions.

Cumulonimbus

Thunderstorm cloud

Contains severe turbulence, icing, lightning, hail, and possibly tornadoes. Avoid by at least 20 NM.

Clear Ice (Glaze Ice)

Smooth, transparent ice — the most dangerous type

Forms when large supercooled water droplets freeze slowly on contact in temperatures near 0°C to -10°C. Adheres strongly, is hard to see, and drastically changes the airfoil shape. Common in freezing rain or large-droplet clouds (SLD conditions). Even FIKI systems can struggle with heavy clear icing.

Clear

No clouds below 12,000 ft (automated observation)

Cold Front

Advancing colder air, usually with a wind shift and a line of showers or storms

Colder, denser air undercuts warmer air ahead of it, forcing it up sharply. Expect a wind shift (commonly to the northwest), a temperature and dewpoint drop, and often a narrow line of showers or thunderstorms along the boundary itself. Passage is usually quick, and conditions behind are frequently clearer but gustier. Timing matters more than position for flight planning — an hour of difference can put the line over your departure airport or well past it.

Convective Activity

Thunderstorms and related weather

Weather driven by vertical air motion. Includes thunderstorms, rain showers, and gusty winds.

Center Weather Advisory

Short-term forecast for hazardous weather

Issued by Air Route Traffic Control Centers for conditions affecting their airspace. Valid for up to 2 hours.

Density Altitude

Pressure altitude corrected for non-standard temperature

The altitude the aircraft 'thinks' it's at based on air density. High DA = thinner air = degraded performance (longer takeoff rolls, reduced climb rates, lower service ceilings). Hot, high, and humid conditions raise DA.

Dryline

A sharp moisture boundary, a common severe-storm trigger on the Plains

Separates moist Gulf air from dry desert air, most often over the southern and central Plains in spring and early summer. There may be little temperature change across it, but the dewpoint can fall 30°F or more in a few miles. It typically shifts east through the afternoon and retreats west overnight, and storms that fire along it can be severe and fast-moving.

European Centre for Medium-Range Weather Forecasts

World's leading global weather model (9 km)

The ECMWF IFS (Integrated Forecasting System) is widely regarded as the most skillful global NWP model. Runs twice daily at 9-km resolution with forecasts out to 15 days. Particularly strong for moisture, upper-level dynamics, and medium-range trends — an independent check on NOAA models (HRRR/GFS).

Eddy Dissipation Rate

Objective measure of turbulence intensity

Unlike subjective PIREPs, EDR is measured by aircraft sensors and provides a standardized turbulence scale. Light: 0.10–0.20, Moderate: 0.20–0.35, Severe: 0.35–0.50, Extreme: >0.50. Lower numbers = smoother ride.

Explosive-Gun Sounding

The cap is essentially gone — storms can fire with no trigger

CAPE ≥ 2,500 J/kg with CIN < 50 J/kg, or any CAPE ≥ 1,000 J/kg with CIN < 25 J/kg — the most dangerous of the three tiers. The suppressing cap has essentially eroded, so convection can initiate on its own with little or no external trigger. Expect rapid intensification and limited warning time.

Explosive-Gun Sounding

The cap is essentially gone — storms can fire with no trigger

CAPE ≥ 2,500 J/kg with CIN < 50 J/kg, or any CAPE ≥ 1,000 J/kg with CIN < 25 J/kg — the most dangerous of the three tiers. The suppressing cap has essentially eroded, so convection can initiate on its own with little or no external trigger. Expect rapid intensification and limited warning time.

Few

1/8 to 2/8 sky coverage (12-25% clouds)

Fog

Visibility <5/8 SM due to water droplets

Flight Into Known Icing

Aircraft certified for icing conditions

Equipped with de-ice or anti-ice systems (boots, heated surfaces, TKS fluid). Allows legal flight into forecasted icing — but doesn't make you invincible. FIKI has limits, especially in severe icing or SLD conditions.

Flight Level

Altitude in hundreds of feet (FL180 = 18,000 ft)

Used for altitudes above 18,000 ft MSL. Based on standard pressure (29.92 inHg).

From

Conditions changing at specified time

Freezing Level

Altitude where temperature reaches 0°C (32°F)

Important for icing forecasts. Icing typically occurs between the freezing level and -20°C.

Freezing Rain

Rain that freezes on contact with cold surfaces

Extremely hazardous for aircraft. Can cause rapid ice accumulation even on FIKI-equipped aircraft.

Graphical AIRMET

Graphical weather advisory updated every 3 hours

Provides forecasts for IFR, mountain obscuration, turbulence, icing, freezing level, and low-level wind shear in a graphical format.

G-AIRMET Sierra — IFR/Mountain Obscuration

Advisory for IFR conditions or mountains hidden by weather

Warns of ceilings below 1,000 ft, visibility below 3 miles, or mountain obscuration due to clouds, precipitation, or fog. Critical for VFR pilots — these areas require IFR capability.

G-AIRMET Tango — Turbulence

Advisory for moderate turbulence or low-level wind shear

Warns of moderate turbulence (surface to FL450) or non-convective low-level wind shear below 2,000 ft AGL. Plan for a bumpy ride and secure loose items.

G-AIRMET Zulu — Icing & Freezing Level

Advisory for moderate icing conditions

Warns of moderate icing and provides freezing level information. If your aircraft is not FIKI-equipped, avoid the icing layer. Even FIKI aircraft should monitor accumulation closely in G-AIRMET Zulu areas.

Global Forecast System

NOAA's primary global weather model

Runs 4 times daily, providing forecasts out to 16 days. The backbone of most weather predictions. Good for big-picture trends but coarser resolution (~13 km).

WX Score

Former name for WX Score

Calculated based on weather conditions relative to your personal minimums and aircraft capabilities. Higher is better.

Graphical Turbulence Guidance Nowcast

Real-time EDR-based turbulence assessment from the FAA and NCAR

GTGN fuses in-situ EDR reports from equipped aircraft, converted PIREPs, satellite-derived turbulence indicators, radar convective signatures, and NWP background fields into a unified turbulence severity grid. Updated every 15 minutes across CONUS at 52 vertical levels. PlaneWX uses GTGN as the primary turbulence source for departures within approximately 2 hours; beyond that, NWP model wind shear analysis takes over. GTGN is the same product used by airline dispatchers.

Hair-Trigger Gun Sounding

The cap is eroding — only weak forcing is needed to fire storms

CAPE ≥ 1,000 J/kg with a weakening cap (CIN 25–50 J/kg), one step more dangerous than a loaded-gun profile. Minor forcing — afternoon heating, a terrain upslope, or a light boundary — is enough to trigger convection, and development can be rapid once it starts.

Hair-Trigger Gun Sounding

The cap is eroding — only weak forcing is needed to fire storms

CAPE ≥ 1,000 J/kg with a weakening cap (CIN 25–50 J/kg), one step more dangerous than a loaded-gun profile. Minor forcing — afternoon heating, a terrain upslope, or a light boundary — is enough to trigger convection, and development can be rapid once it starts.

High-Resolution Rapid Refresh

NOAA's highest-resolution hourly weather model (3 km)

Updates every hour with 3-km resolution — the most detailed model available. Excellent for icing, cloud layers, and convection in the 0–18 hour window. Its fine grid captures terrain effects and local weather features that coarser models miss.

Haze

Reduced visibility due to fine particles

Aircraft Icing

Ice accumulation on aircraft surfaces

Degrades lift, increases drag, and adds weight. Trace, Light, Moderate, and Severe intensities.

Instrument Flight Rules

Ceiling 500-999 ft OR visibility 1-3 SM

Requires instrument rating and IFR-equipped aircraft. Flight by reference to instruments.

Instrument Meteorological Conditions

Weather requiring instrument flight

Low IFR

Ceiling <500 ft OR visibility <1 SM

Very poor conditions. Even experienced instrument pilots exercise extreme caution.

Low-Level Wind Shear

Rapid wind change below 2,000 ft AGL

Can cause sudden airspeed changes during takeoff and landing. Especially dangerous near thunderstorms.

Loaded-Gun Sounding

Energy is building under a cap that hasn't broken yet

CAPE ≥ 1,000 J/kg with a cap still present (CIN 50–150 J/kg). The atmosphere has real energy for thunderstorms, but it's held down for now. A trigger — peak afternoon heating, a front, or a dryline — could break the cap and let storms develop. Not an active threat yet, but worth monitoring through the afternoon.

Loaded-Gun Sounding

Energy is building under a cap that hasn't broken yet

CAPE ≥ 1,000 J/kg with a cap still present (CIN 50–150 J/kg). The atmosphere has real energy for thunderstorms, but it's held down for now. A trigger — peak afternoon heating, a front, or a dryline — could break the cap and let storms develop. Not an active threat yet, but worth monitoring through the afternoon.

Meteorological Aerodrome Report

Current weather observation at an airport

Issued hourly (or more frequently when conditions change significantly). Contains wind, visibility, clouds, temperature, dewpoint, and altimeter setting.

Mixed Ice

Combination of clear and rime ice

Occurs when temperature and droplet size vary within the icing layer. May alternate between clear and rime characteristics. Can be especially difficult to remove because the irregular surface traps additional moisture.

Model Output Statistics

Statistical post-processing of model forecasts

Improves raw model output using historical observations. GFS MOS provides point forecasts for airports.

Mean Sea Level

Height measured from average sea level

Mountain Obscuration

Mountains hidden by clouds, fog, or precipitation

Extremely dangerous for VFR flight in mountainous terrain. Wait for conditions to improve.

Marginal VFR

Ceiling 1,000-3,000 ft OR visibility 3-5 SM

Reduced but flyable conditions. Extra caution required. Some restrictions may apply.

North American Mesoscale Forecast System

Regional weather model covering North America (12 km)

Runs 4 times daily with 12-km resolution. Good for precipitation type, frontal boundaries, and icing forecasts out to 84 hours. Bridges the gap between HRRR (very detailed, short range) and GFS (coarser, long range).

National Blend of Models

NOAA's most accurate hourly weather forecast

Combines multiple weather models into a single, statistically calibrated forecast. Provides hour-by-hour predictions for visibility, clouds, wind, and precipitation probability.

Nautical Miles

Unit of distance (1 NM = 1.15 SM)

Notice to Air Missions

Official notice of flight hazards or changes

Alerts pilots to temporary hazards, closed runways, airspace restrictions, and other important information.

National Weather Service

U.S. government weather forecasting agency

Outside Air Temperature

Current air temperature at flight altitude

Critical for icing assessment: icing occurs between 0°C and -20°C. Clear ice forms near 0°C to -10°C (most dangerous), rime ice forms -10°C to -20°C. Below -20°C, air is typically too cold for significant icing.

Occluded Front

A cold front has overtaken a warm front, wrapping the system up

Occurs late in a low-pressure system's life when the faster cold front catches the warm front. Mixes the characteristics of both — broad layered cloud and precipitation like a warm front, with a sharper wind shift like a cold front. Usually indicates the parent low is mature and beginning to weaken.

Outflow Boundary

The rain-cooled gust front spreading out from a thunderstorm

Cold air descending from a thunderstorm spreads along the surface and can travel well away from the parent storm, long outliving it. Brings an abrupt wind shift and gusts, which matters for crosswind and low-level shear on approach, and can trigger fresh storms where it collides with another boundary. Short-lived and local, so timing is everything.

Overcast

8/8 sky coverage (100% clouds)

Pressure Altitude

Altitude above the standard datum plane (29.92 inHg)

The altitude read on the altimeter when set to 29.92 inHg. Used for aircraft performance calculations and flying above 18,000 ft.

Pilot Report

In-flight weather observation from a pilot

Real-time reports of actual conditions encountered including turbulence, icing, cloud tops/bases, and visibility. Invaluable for current conditions.

Probability

Percent chance of conditions occurring

Rain

Liquid precipitation

Relative Humidity

Percentage of moisture in the air relative to saturation

RH ≥ 80% indicates cloud / visible moisture (highlighted in the icing table). RH ≥ 60% with icing-range temperatures (0°C to -20°C) suggests enough moisture for ice to form. Higher RH in the icing temp range = higher icing probability.

Richardson Number

Atmospheric stability index — lower values mean more turbulence potential

Ri measures the balance between thermal stability (which suppresses turbulence) and wind shear (which generates it). Ri < 0.25: dynamically unstable, turbulence likely. Ri 0.25–1.0: transitional, turbulence possible. Ri > 1.0: stable, turbulence suppressed regardless of shear.

Richardson Number (Ri)

Atmospheric stability indicator for turbulence forecasting

A dimensionless ratio of thermal stability to wind shear. Ri < 0.25 = dynamically unstable (conditions support turbulence), 0.25–1.0 = marginally stable, Ri ≥ 1.0 = stable (turbulence suppressed). Ri indicates whether turbulence CAN develop — severity comes from shear magnitude. PlaneWX calculates this from multi-model pressure-level wind and temperature profiles.

Rime Ice

White, rough, opaque ice — lighter but still hazardous

Forms when small supercooled water droplets freeze rapidly on contact in colder temperatures (-10°C to -20°C). Builds forward-facing on leading edges, creating a rough surface. Less dense than clear ice but disrupts airflow. More easily removed by de-ice equipment. Common inside stratiform clouds.

Scattered

3/8 to 4/8 sky coverage (37-50% clouds)

Surface

Ground level

Rain Showers

Brief, intermittent rain

Significant Meteorological Information

Weather advisory for conditions hazardous to ALL aircraft

Issued for severe icing, severe/extreme turbulence, dust storms, volcanic ash, and convective activity (thunderstorms).

Sky Clear

No clouds observed

Supercooled Large Droplets

Large water droplets below freezing — extremely hazardous icing

Droplets larger than 50 microns that remain liquid below 0°C. When they strike your aircraft, they run back past protected surfaces before freezing, causing ice to form BEHIND boots and heated areas where de-ice systems can't reach. FIKI (Appendix C) does not protect against supercooled large droplets; the SLD certification basis, Appendix O, is confined to transport-category aircraft, so assume your aircraft is not approved for SLD unless its AFM says so. CIP and IFI report an uncalibrated SLD potential index — not a probability — where a nonzero value means large droplets may be present. Associated with freezing rain and freezing drizzle. A warm nose inversion is a common setup for SLD conditions.

Statute Miles

Unit of visibility measurement

Snow

Frozen precipitation

Special METAR

Unscheduled weather observation due to significant changes

Issued when weather conditions change significantly between regular hourly observations.

Stationary Front

A boundary that has stalled, holding weather in place for days

Neither airmass is displacing the other, so the boundary sits roughly still. The significance for planning is persistence: clouds, precipitation and any convection along it can repeat in the same area for days rather than passing through. Waiting for a stationary front to clear is often the wrong plan, because it may not.

Synoptic Intelligence™

AI-synthesized regional weather summary

PlaneWX analyzes Area Forecast Discussions from multiple NWS offices to create a regional weather narrative for your route.

Terminal Aerodrome Forecast

Weather forecast for a 5-mile radius around an airport

Typically valid for 24-30 hours. Updated every 6 hours. Contains expected winds, visibility, clouds, and weather phenomena.

True Airspeed

Actual speed through the air (corrected for altitude/temp)

Your real speed through the air mass, correcting for thinner air at altitude. Add wind to get ground speed: TAS ± headwind/tailwind = ground speed.

Towering Cumulus

Rapidly growing cumulus cloud

Precursor to thunderstorms. Can develop into CB within 15-30 minutes.

Temporary

Conditions expected temporarily (<1 hour at a time)

Temporary Flight Restriction

Airspace temporarily restricted

Issued for VIP movement, emergencies, sporting events, and other special circumstances.

TKS Ice Protection System

Fluid-based de-icing system (glycol weeping panels)

Pumps glycol-based fluid through laser-drilled panels on wing and tail leading edges. The fluid prevents ice from bonding and sheds existing ice. Finite fluid supply — total icing exposure time determines if you have enough fluid for the flight.

Trough

An elongated area of low pressure — a focus for lift, without a true airmass change

Not a front: there is no significant airmass boundary, so there is no meaningful "ahead of it" or "behind it" for planning. It still provides the lift that can organise showers and storms, which is why a forecast office will discuss one. PlaneWX treats a trough as weather to be aware of rather than a passage to time a departure around.

Thunderstorm

Storm with lightning and thunder

Contains multiple hazards: turbulence, icing, hail, wind shear, and microbursts.

Thunderstorm with Rain

Active thunderstorm with precipitation

Turbulence

Irregular air motion causing aircraft bumps

Intensities: Light (slight strain on seat belts), Moderate (definite strain), Severe (violent, momentary loss of control), Extreme (impossible to control).

Visual Flight Rules

Ceiling >3,000 ft AND visibility >5 SM

Good flying weather. Pilots can navigate by visual reference to the ground.

Visual Meteorological Conditions

Weather allowing visual flight

Warm Front

Advancing warmer air, with widespread low ceilings ahead of it

Warmer air rides up over retreating cold air on a shallow slope, producing a broad area of stratiform cloud, steady precipitation, and low ceilings and visibility well ahead of the surface position — often hundreds of miles ahead. Moves more slowly than a cold front, so the IFR conditions linger. In winter the precipitation ahead of it is a classic freezing-rain setup.

Warm Nose Inversion

A layer of above-freezing air sandwiched between freezing layers aloft

Picture a temperature 'sandwich': freezing air on top, a warm layer in the middle (the 'nose'), and freezing air below. Snow falls from above, melts in the warm layer, then the liquid drops re-enter freezing air below. These supercooled drops are large (SLD) and freeze on contact with your aircraft. This is the classic setup for freezing rain — the most dangerous icing condition. If a warm nose is detected along your route, exercise extreme caution, especially during descent through the warm-to-cold transition zone.

Weather Forecast Office

Local NWS office responsible for a geographic area

Each office produces forecasts and warnings for its own area, and writes the Area Forecast Discussion explaining its reasoning. PlaneWX reads all 123 offices that issue one, covering the continental US, Alaska, Hawaii, Puerto Rico, the US Virgin Islands, Guam and American Samoa, and shows you what the office responsible for your airport said.

Wind Shear

Sudden change in wind speed or direction over a short distance

Vertical wind shear is measured in knots per 1,000 ft. Values > 12 kt/1000 ft indicate light turbulence potential; > 20 kt/1000 ft moderate; > 30 kt/1000 ft severe. Low-level wind shear (LLWS) below 2,000 ft AGL is particularly dangerous during takeoff and landing.

Weather Prediction Center

NOAA center for precipitation and severe weather forecasts

Provides national-scale guidance on fronts, precipitation, and significant weather systems.

WX Score

PlaneWX flight safety rating (0-100)

Calculated based on weather conditions relative to your personal minimums and aircraft capabilities. Higher is better.