Glossary
The radar words, in plain language
Radar has a vocabulary problem: the products are named after the physics rather than after what you see. Here is the whole list, one page each, no meteorology degree assumed.
- CAPE How much energy is available for a thunderstorm to rise. More CAPE, taller storms.
- CC (correlation coefficient) Whether everything in the beam looks alike. High for rain, low for a mix — hail, melting snow, or tornado debris.
- dBZ The unit of reflectivity. Roughly: how much is coming back.
- Dealiasing Undoing the fold that happens when wind is faster than the radar can measure directly, so a fast couplet reads correctly.
- Dual-pol The radar sending both horizontal and vertical pulses, which is what makes CC and ZDR possible. Rolled out across the network around 2012.
- Hook echo The comma-shaped notch on the back edge of a supercell where the rear-flank downdraft wraps precipitation around the mesocyclone — the classic radar look of a tornadic storm.
- Level 2 / Level 3 Level 2 is the raw volume, every gate of every sweep. Level 3 is the smaller set of processed products the NWS publishes from it.
- MESH Maximum estimated hail size, computed from the storm's vertical structure.
- MRMS The national grid that stitches every radar in the country into one picture, updated every couple of minutes.
- NEXRAD The US network of 150-odd weather radars. Each has a four-letter ID like KTLX.
- Reflectivity (Z) How much energy the echo sends back. Where the rain is.
- SRH (storm-relative helicity) How much spin the low-level wind has for a storm to use.
- Storm-relative velocity Velocity with the storm's own travel subtracted, so rotation stops hiding inside the storm's motion.
- TDS (tornado debris signature) High reflectivity, a velocity couplet, and a hole in CC in the same place: the radar seeing lofted debris.
- Tilt One sweep of the radar at one elevation angle. A volume is a stack of them.
- VCP The scanning pattern the radar is running. Storm modes sweep faster and use more tilts than clear-air modes.
- Velocity (V) Motion toward or away from the radar along the beam. Green toward, red away.
- VIL How much water the storm is holding, integrated through its depth.
- ZDR (differential reflectivity) How flat the things in the beam are. Positive for raindrops, near zero for tumbling hail.