Joplin, 2011

At 5:34 pm on a Sunday afternoon, a multiple-vortex EF5 crossed the southern edge of Joplin, Missouri, from west to east. It killed 158 people and destroyed about a third of the city.

What the radar shows. Springfield’s WSR-88D was watching from roughly 90 miles away, so the beam was already thousands of feet above the ground by the time it reached Joplin — and it still lit up. The link opens on correlation coefficient, the dual-polarization product that measures how alike the targets in a radar bin are. Rain is uniform and reads high. Snapped lumber, insulation and roof decking lofted into the air are not, and read as a hole in the field sitting exactly under the velocity couplet: a tornadic debris signature. On this volume there is no ambiguity about whether something is on the ground.

Switch to velocity and the couplet is unmistakable; switch to reflectivity and you get the hook the app is named after.