Weather & Aviation Glossary

Area Forecast Discussion

Forecaster's narrative explaining the weather situation

Written by NWS meteorologists, explaining their reasoning and confidence levels. Great for understanding the 'why' behind forecasts.

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)

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.

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.

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).

GO Score

PlaneWX flight safety rating (0-100)

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

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.

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.

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. Even FIKI-certified aircraft may not be approved for SLD. 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.

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.

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 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

There are 122 WFOs across the U.S., each producing forecasts and warnings for their region.

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.