Live radar / Reflectivity

Live radar reflectivity

Rain, snow and hail across the country now, from every radar in the national network, a new picture every five minutes. How to read it.

Radar on the map

The rings are the storms the radars detect, the larger ring a severe one; each line runs to where its storm will be in an hour if it keeps its present speed and heading. Rotation and hail are what the radars detect and the analysis measures, not warnings: the official watches and warnings in force are drawn on the map in their own colours and set out in full under Alerts.

What the radar map shows

How strongly rain, snow and hail send the radar’s pulse back, from every radar in the national network joined into one picture. The stronger the return, the heavier the fall. Each picture is checked against the national analysis, and echo it does not confirm is left out.

How to read it

10 to 20 dBZ: Drizzle, flurries or the faint edge of a shower.
20 to 35 dBZ: Light rain or steady snow.
35 to 45 dBZ: Moderate rain, the body of a storm.
45 to 55 dBZ: Heavy rain, a thunderstorm’s core.
Over 55 dBZ: Torrential rain, and hail likely; over 65, large hail.

What it is for

Watch a storm come in and judge when it will reach a town.
See the shape of a storm: a line, a bow that pushes damaging wind ahead, or a hook at a supercell’s tail.
Tell where rain turns to snow along a winter storm.
Check the road ahead before a drive.

How it is made

Every radar in the national network, more than 150 of them, sweeps the sky every few minutes, tilting up through a set of angles. The sweeps are joined into one picture of the country on a grid about a third of a mile across, each place taking the lowest beam that sees it clearly. Each picture is checked against the national analysis before it is drawn, and echo it does not confirm (birds, insects, ground clutter, the beam bent back to the ground) is left out.

A picture every 5 minutes, played as a loop of 1 to 24 hours. As data: /radar/now.json, the times of its frames.