Rotation, live radar analysis

What the rotation map shows

How sharply the wind turns in the lowest 6,500 ft of each storm, in mph across a mile, from the radars’ measure of the wind blowing toward and away from them. A tight counterclockwise turn is the rotation a storm needs to bring a tornado, drawn in warm colours; a clockwise turn is drawn in blue.

How to read it

Under 20 mph a mile

Little turning.

20 to 40

A storm beginning to rotate.

40 to 80

Strong rotation; a tornado possible.

Over 80

Intense rotation, the kind under a strong tornado.

Blue

The wind turning clockwise, often beside a counterclockwise turn or where the wind spreads out of a storm.

What it is for

• See which storms are rotating, and how tightly.
• Follow the rotation inside a line of storms, where a tornado can form quickly.
• Read with the warnings: a tornado warning drawn round a strong turn.

How it is made

The storm analysis joins every radar in the national network every two minutes into one picture of the country on a grid about two thirds of a mile across, cleaned of ground clutter, birds and insects, and measures from it what no single radar shows: the largest hail, the turning of the wind, how high a storm’s heavy echo reaches above the freezing level, and what is falling, read with the temperature of the air. Storm Station keeps it every ten minutes for a day, the gauged rain and the radars’ view each hour, and draws it at every zoom from the analysis’s own grid.

A frame every 10 minutes, played as a loop of 1 to 24 hours. As data: /radar/now.json?show=azshear, the times of its frames.