How to read weather radar for tornadoes

The National Weather Service, the FAA, and the Department of Defense jointly run a network of 160 Doppler weather radars. Forecasters use them to spot storms that could produce tornadoes. You can look at the same kinds of images. Here is what to look for, and why radar should never replace an official warning.

Reflectivity: where the rain and hail are

Reflectivity is the classic radar map. It shows how much energy bounces back from raindrops, hail, and other objects. It is measured in dBZ:

Check the dBZ legend on whatever radar map you use.

The hook echo. A hook echo is a hook-shaped curl in the reflectivity pattern. It is a good sign the storm is rotating, and it is common in severe supercell storms. Tornadoes in supercells are often found at the tip of the hook. Two cautions: real hooks are rarely as clear as the textbook pictures, and a composite reflectivity image can hide the hook. A base reflectivity image shows it better.

Velocity: which way the wind is blowing

Doppler radar can also measure how fast things are moving toward or away from the radar.

• Green means moving toward the radar.
• Red means moving away from the radar.

The radar only sees motion along its beam. Wind blowing straight across the beam shows up as zero.

The velocity couplet. When a storm spins, part of it moves toward the radar and part moves away. On the velocity map, this looks like a tight patch of bright green right next to bright red. This pair is called a couplet. A strong, tight couplet means intense rotation and a possible tornado. Radar does not see the tornado itself. It sees the rotation.

Fast-moving storms can mask a couplet. A view called storm-relative velocity subtracts the storm’s own motion, which can make the rotation easier to spot.

Correlation coefficient: signs of debris

Since about 2012, NWS radars have used dual-polarization, which adds new views. One is correlation coefficient, or CC. It shows how alike the objects in the beam are, on a scale from 0 to 1.

• Rain or snow alone gives high values, above about 0.97.
• Hail, or a mix of rain and snow, gives moderate values, about 0.80 to 0.97.
• Things that are not weather, such as birds, bugs, or smoke, give low values, below about 0.80.

When a tornado lifts debris into the air, it can create a tornado debris signature: a sharp drop in CC at the same spot as a strong velocity couplet. Both must line up. When they do, it is strong evidence a tornado is on the ground. This kind of radar evidence can lead the NWS to mark a warning as tornado observed.

A debris signature confirms a tornado that is already happening. It does not predict one. Not every tornado shows one, because debris has to be lifted high enough for the beam to catch it.

The limits of radar

• The beam rises with distance. About 20 miles from the radar, the center of the beam is roughly 1,300 feet above the ground. About 98 miles away, it is about 11,600 feet up. Far from a radar, the beam passes over the low levels of the storm, which makes tornadoes harder to detect. Radar also sees little below the cloud base except close to the radar.
• The beam spreads out. It widens about 1,000 feet for every 10 miles, so it is over 2 miles wide at 120 miles. Small features can blur together.
• Some tornadoes don’t show up well. Some form without a clear rotation signature, and heavy rain around a storm can make its hook harder to pick out.

Radar is not a warning

Radar images can help you follow a storm. But a strong couplet means a tornado is possible, not certain. And a quiet-looking image does not mean you are safe. If the NWS issues a warning for your area, act on the warning. Do not wait to confirm it on radar yourself.

See current warnings on the live tornado warning map.