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.
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.
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.
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.
Velocity: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
How Tornadoes Form: Types, EF Scale, and Safety
Learn how tornadoes form from supercell thunderstorms, the Enhanced Fujita scale, types of tornadoes, and critical safety steps to protect yourself.
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.
Weather Education
Ice Storm vs Freezing Rain vs Sleet: What's the Difference?
Learn the key differences between ice storms, freezing rain, and sleet. Understand how each forms, what they look like, and how to stay safe during winter precipitation events.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
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.
Feature
Professional Radar
Explore high-resolution radar, premium lightning, and satellite layers built for serious weather tracking.
Guide
Radar Beam Height Explained
Learn why radar can miss low-level rain, snow, and storm details depending on distance from the radar site.
Glossary
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm.
Explore Other Terms
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.
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
Probability of Precipitation
Probability of precipitation (PoP) is the chance that a given point receives measurable precipitation, at least 0.01 inch, during the forecast period. A 40% chance of rain means a 4 in 10 chance of at least 0.01 inch at your location. It does not mean rain over 40% of the area or for 40% of the time.
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.
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