What is NEXRAD? #
NEXRAD (Next Generation Weather Radar) is the U.S. network of 159 WSR-88D Doppler radars operated jointly by the NWS, FAA, and DoD. It provides the radar data behind nearly every warning issued in the U.S.
Radar Fundamentals
Before you can read a hook echo, you need to know what the radar is actually measuring. This page covers how NEXRAD works: reflectivity, velocity, scan strategies like SAILS and MRLE, beam geometry, and the artifacts that fool beginners. It pairs with the Radar Signatures & Dual-Pol page, and you can put all of it to use on our own radar at radar.xtremewx.com or with Snowball’s radar commands in the XWD Discord.
What is Dual-Pol Radar? #
Dual-Polarization radar transmits both horizontal and vertical pulses, enabling products like ZDR, CC, and KDP. NEXRAD was upgraded to dual-pol in 2011–2013, revolutionizing precipitation typing and tornado debris detection.
What is Reflectivity? #
Reflectivity measures how much energy is returned to the radar from precipitation — higher values mean larger/denser targets. The classic radar image you see is usually base reflectivity (in dBZ).
What is Velocity? #
Velocity measures how fast precipitation is moving toward or away from the radar. Used to identify rotation, convergence, and wind speed — the most important product for spotting tornadoes.
What is Storm Relative Velocity? #
Storm Relative Velocity (SRV) subtracts storm motion from base velocity, isolating rotation from forward motion. Makes mesocyclones and tornado-scale circulations much easier to identify.
What is Spectrum Width? #
Spectrum Width (SW) is a radar product that measures the variability of velocities within a single radar pixel. High SW indicates turbulent, mixed motion — often inside tornadic circulations, gust fronts, or strong shear zones. It's a supporting product alongside velocity and CC for confirming rotation and debris.
What is the Base Scan? #
The base scan is the lowest elevation tilt of a NEXRAD radar — typically 0.5° above horizontal. It's the closest look at what's happening near the ground, which makes it the primary scan for spotting low-level rotation, hook echoes, and debris signatures. Higher tilts (0.9°, 1.5°, 1.8°, etc.) sample progressively higher in the storm.
What is SAILS? #
SAILS (Supplemental Adaptive Intra-Volume Low-Level Scan) is a NEXRAD feature that inserts extra low-level scans into a radar volume coverage pattern, giving forecasters faster updates on what's happening near the ground — often where tornadoes form. MRLE (Multiple Elevation Scan Option) does the same thing for slightly higher tilts. Both reduce update times from 4-5 minutes to as little as 2 minutes.
What is MRLE? #
MRLE (Multiple Elevation Scan Option) is a NEXRAD scanning enhancement that adds extra low-tilt scans inside a volume, similar to SAILS but at a slightly higher elevation. It speeds up radar updates in the lowest few thousand feet, which is critical during severe weather and tornado warnings.
What is Super-Resolution Radar? #
Super-resolution is a NEXRAD upgrade providing 0.5° azimuthal resolution and 250m gate spacing on the lowest tilts (vs. 1° and 1km previously). It dramatically improves the ability to detect small-scale features like tight velocity couplets, debris signatures, and inflow notches.
What is Radar Beam Height? #
Radar beam height is how high above the ground the radar beam is at a given distance from the station. Because of Earth's curvature and beam tilt, even the lowest scan is several thousand feet up by 100 miles out. That's why low-level features (rotation, surface boundaries, weak tornadoes) often go undetected at long range — the beam is overshooting them.
What is the Cone of Silence? #
The cone of silence is the area directly above a NEXRAD radar that the radar can't see, because even its highest tilt (~19.5°) doesn't quite point straight up. Tornadoes that pass directly over a radar site often vanish from the radar's view in the cone of silence. The fix is to use a neighboring radar.
What is Ground Clutter? #
Ground clutter is spurious radar return from buildings, terrain, and other stationary objects near the radar site. It appears as a halo of high reflectivity near the radar center on low-tilt scans. Modern NEXRAD uses clutter filtering algorithms, but anomalous propagation can cause it to break through.
What is Anomalous Propagation? #
Anomalous Propagation (AP) is a radar artifact where the beam bends toward the surface under temperature inversion conditions, producing false returns that look like precipitation. Common in the morning/evening when inversions form. Dual-pol CC values near 1.0 and lack of weather context help identify AP clutter vs real precipitation.
What is Range Folding? #
Range folding (second-trip echoes) occurs when a radar pulse reflects off a distant target beyond the unambiguous range and the return arrives during the next pulse interval — appearing at the wrong location as smeared radial artifacts. Looks like bands of fake precipitation. Dual-PRF processing helps mitigate it on modern NEXRAD.
What is Radar Aliasing? #
Aliasing occurs when a target's velocity or range exceeds the radar's maximum unambiguous value. Velocity aliasing appears as an abrupt wrap-around of colors (e.g., strong green suddenly appearing next to strong red). Meteorologists 'dealias' velocity data to reveal the true wind field — crucial for tornado detection.
What is a Clear-Air Echo? #
Clear-air echoes (sometimes called 'angel echoes') are radar returns from insects, birds, bats, or refractive index gradients in the atmosphere — not precipitation. The classic sunrise 'ring of fire' on radar is insects lofted by morning convection. Clear-air echoes can reveal boundaries, sea breezes, and outflow boundaries invisible in reflectivity.
What are Echo Tops? #
Echo Tops show the maximum altitude of detectable precipitation in a storm. Higher tops (>50,000 ft) generally indicate stronger updrafts and severe potential.
What is MRMS? #
MRMS (Multi-Radar Multi-Sensor) is a NWS system that merges all NEXRAD radar data with surface observations and model output into a seamless, high-resolution mosaic updated every 2 minutes. It produces gap-filled national composites, rotation tracks, severe weather signatures, and precipitation estimates — including the AzShear rotation product.
What is a B-Scan? #
A B-scan is a radar display mode showing a vertical cross-section of a storm along a selected radial. Instead of top-down reflectivity, you see height vs. range — useful for diagnosing bounded weak echo regions (BWER), overshooting tops, and hail shafts.
What is a DOW? #
A DOW (Doppler on Wheels) is a mobile, truck-mounted research radar developed at the Center for Severe Weather Research. Because it can drive right up to storms, DOWs sample tornadoes at distances impossible for NEXRAD — sometimes catching wind speeds over 300 mph inside violent tornadoes (Bridge Creek-Moore 1999 being the most famous example).
What is RaXPol? #
RaXPol (Rapid X-band Polarimetric radar) is a mobile dual-pol research radar based at the University of Oklahoma. Its rapid-scan capability — updating tornado-scale volumes in seconds rather than minutes — has given researchers an unprecedented look at tornado evolution, multiple-vortex structure, and debris dynamics.