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.
More Aviation Weather Terms
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.
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.
Explore Other Terms
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.
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.
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.
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Learn SIGMET in Context
These explainers and product surfaces connect this definition to real-world forecasting and decision-making.
Weather Education
GFS vs HRRR vs NAM: Which Weather Model Should You Trust?
Learn the differences between GFS, HRRR, NAM, and the Euro model. Understand which forecast model to trust for different situations and how weather enthusiasts can interpret model data.
Guides
METAR Codes Decoded: A Pilot's Guide to Reading Aviation Weather
Learn how to read and decode METAR weather reports. Understand aviation weather codes, visibility, cloud layers, and all the abbreviations pilots need to know.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
Lightning Safety Guide
Know when lightning becomes dangerous, how far away it can strike, and when to move indoors.
Feature
Volcano Monitoring
Track US volcano activity, USGS alert levels, and aviation color codes.
Guide
What Causes Thunder and Lightning?
How lightning forms through charge separation, what causes thunder, and how to stay safe.
Feature
Aviation Weather
Track METARs, TAFs, and airport conditions in a pilot-friendly layout.
See SIGMET in Action
WeatherAI brings weather concepts to life with professional radar, real-time alerts, and AI-powered forecasts that explain what the weather means for you.