Live radar reflectivity

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.