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.
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.
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.
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.
Reflectivity: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Understanding Flash Floods: Causes, Warnings, and Safety
Learn what causes flash floods, how to spot the risk on radar, the difference between watches, warnings, and emergencies, and critical safety steps that save lives.
Weather Education
What Is a Derecho? The Inland Hurricane Explained
Learn what a derecho is, how it differs from tornadoes and hurricanes, why these long-lived windstorms are so destructive, and how to stay safe when one approaches.
Guides
How to Read Satellite Imagery: A Complete Guide to Weather From Space
Master visible, infrared, and water vapor satellite imagery. Learn the science behind each channel and how meteorologists use them to track storms.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
Weather Radar Colors Explained
Understand what green, yellow, red, and purple radar returns usually mean in real-world storms.
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.
Guide
What a 30% Chance of Rain Means
What the probability of precipitation measures, why hourly and daily chances differ, and what the number leaves out.
Glossary
Hail
Balls or irregular lumps of ice produced in severe thunderstorms.
Explore Other Terms
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
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.
Earthquake Early Warning
A system that detects the fast P-wave near an earthquake's origin and issues an alert before the slower, more damaging shaking arrives, providing seconds to tens of seconds of notice. It is detection, not prediction: the earthquake has already begun. Public systems operate in regions including the U.S. West Coast (ShakeAlert), Japan, and Mexico.
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.
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.
Cold Front
A cold front is the boundary where a cooler, denser air mass is advancing and replacing warmer air. The cold air wedges under the warm air and forces it upward, which often produces a band of showers or thunderstorms, followed by a wind shift and cooler, drier air.
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