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.
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.
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.
LIFR
LIFR, Low Instrument Flight Rules, is the lowest of the four aviation weather flight categories. It means a ceiling below 500 feet and/or visibility of less than 1 statute mile. Whichever of the two is lower sets the category. The categories above it, in order, are IFR, MVFR, and VFR.
PIREP
A PIREP, or pilot report, is a report of the actual weather an aircraft encounters in flight, made by the pilot or a crew member. PIREPs describe conditions such as turbulence, icing, and cloud layers at a stated location and altitude, which fills the gaps between ground stations with direct observations from the air.
MVFR: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
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.
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.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
Flight Rule Calculator
Classify VFR, MVFR, IFR, and LIFR conditions from ceiling and visibility.
Feature
Aviation Weather
Track METARs, TAFs, and airport conditions in a pilot-friendly layout.
Use Case
Weather App for Pilots
Follow the aviation workflow for METARs, TAFs, ceilings, and crosswinds.
Tool
Cloud Base Calculator
Estimate cloud base height from temperature and dew point spread.
Explore Other Terms
Kp Index
The Kp index is a 0 to 9 scale of global geomagnetic activity, reported in 3-hour intervals from ground-based magnetometers. A Kp of 5 or higher indicates a geomagnetic storm, and the values map onto NOAA's G-scale from G1 (Kp 5) to G5 (Kp 9). The higher the Kp, the farther from the poles the aurora can be seen.
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.
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
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.
Heat Index
The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.
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.
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