MRMS
Multi-Radar Multi-Sensor, an advanced system that combines data from multiple radars, satellites, and surface observations to create a seamless, high-resolution national weather picture updated every 2 minutes.
More Radar & Observation Terms
NEXRAD
Next-Generation Radar, a network of 159 high-resolution Doppler weather radars operated by the National Weather Service. NEXRAD detects precipitation, wind patterns, and severe weather signatures up to 250 miles from each station.
Reflectivity
A measure of how much radar energy is reflected back by precipitation, measured in dBZ (decibels of Z). Higher values indicate heavier precipitation or larger particles. Values above 50 dBZ typically indicate heavy rain or hail.
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.
Echo Tops
The maximum height at which radar detects precipitation in a storm, indicating storm intensity and updraft strength. Higher echo tops generally correlate with stronger storms. Tops exceeding 40,000 feet suggest severe weather potential.
Hook Echo
A distinctive hook-shaped radar signature on the right rear of a supercell thunderstorm, indicating rotation and high tornado probability. Hook echoes form as precipitation wraps around the mesocyclone.
Bow Echo
A bow or crescent-shaped radar signature indicating strong straight-line winds. Bow echoes form when the center of a squall line accelerates ahead of its ends, often producing damaging winds exceeding 75 mph.
Debris Ball
A radar signature showing a concentrated area of debris lofted by a tornado. Debris balls appear as a circular area of high reflectivity collocated with rotation and confirm a tornado is on the ground causing damage.
Doppler Radar
Doppler radar is weather radar that measures the motion of precipitation as well as its location and intensity. It sends out pulses of radio waves and detects the shift in phase of the signal that returns, which shows whether targets are moving toward or away from the radar and how fast. That reveals wind patterns and rotation inside storms.
dBZ
dBZ, short for decibels of Z, is the unit of radar reflectivity: a logarithmic measure of how much energy precipitation returns to the radar. The scale runs from about -35 to +85 dBZ. As a guide, 20 to 40 dBZ is light precipitation, 40 to 50 is moderate, and 50 to 65 is heavy rain or some hail.
Correlation Coefficient
Correlation coefficient (CC) is a dual-polarization radar product that measures how similar the horizontal and vertical radar pulses are after they hit their targets, on a scale from 0 to 1. Uniform rain or snow gives values above 0.97. Mixed precipitation and hail give lower values, and tornado debris typically falls below 0.80.
Skew-T
A Skew-T Log-P diagram is the chart meteorologists use to plot a weather balloon sounding: temperature, dew point, and wind from the ground to the upper atmosphere. Its temperature lines are tilted 45 degrees, which gives the chart its name. A hodograph, often shown beside it, plots how wind speed and direction change with height.
MRMS: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Lake Effect Snow: Why One Town Gets Three Feet and the Next Gets Flurries
What lake effect snow is, how it forms, why Buffalo gets so much of it, why totals change within a few miles, and what a Lake Effect Snow Warning means today.
Weather Education
Radar Beam Height Explained: Why Distance From Radar Matters
Learn how radar beam height affects weather detection. Understand why storms far from radar may appear weaker and how Earth's curvature impacts precipitation detection.
Guides
Weather Radar Colors Explained: How to Read Radar Like a Pro
Learn what weather radar colors mean, from light green to purple. Understand reflectivity, precipitation intensity, and how to interpret radar images.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Feature
Professional Radar
Explore high-resolution radar, premium lightning, and satellite layers built for serious weather tracking.
Guide
How to Read Weather Radar
Learn the basics of radar interpretation so you can tell where storms are headed and how intense they may become.
Use Case
WeatherAI for Storm Chasers
See how WeatherAI supports active storm tracking with radar, severe weather context, and storm-focused workflows.
Guide
El Niño 2026 Explained
Where the current El Niño stands and what a strong event means for hurricane season and the winter ahead.
Explore Other Terms
Relative Humidity
The ratio of the current absolute humidity to the maximum possible humidity at the current temperature, expressed as a percentage. 100% relative humidity means the air is fully saturated. Relative humidity changes with temperature even if moisture content stays the same.
NAM
North American Mesoscale Model, a regional weather model covering North America at 3-12 km resolution and NCEP's short-range workhorse for mesoscale features like fronts, lake-effect snow, and sea breezes. Its successor is the Rapid Refresh Forecast System (RRFS) with its ensemble REFS, and the same changeover ends SREF, HREF, HiresW, and NAM MOS. The National Weather Service notice of October 2, 2026 set the changeover date as November 3, 2026, moved from October 14.
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.
Typhoon
A tropical cyclone with sustained winds of 74 mph or greater in the Western Pacific. Typhoons are meteorologically identical to hurricanes; the different name reflects only their geographic location.
Ice Storm
A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
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