Ask meteorologists: how does radar see tornadoes? | Weather


Q: How do radars see tornadoes?

A: A weather radar consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern. Precipitation scatters these radio waves.

“Reflectivity” is the amount of transmitted power returned to the radar and measured by its receiver. The intensity of this received signal indicates the intensity of the precipitation.

Measuring the time it takes for the radio wave to leave the radar and return tells us how far away the storm is. The direction pointed by the radar locates the storm.

Dirt, plant matter, and other debris thrown into the atmosphere by a tornado on the ground can be detected by radar.


A hook echo is a pattern in a reflectivity image. It looks like a spiral rotating outward clockwise, with the precipitation “thickness” increasing – or, a hook shape. This pattern suggests that the storm is rotating and may produce a tornado. A tornado could be found at the narrow top of the spiral.

Doppler radars measure the speed at which particles in the cloud are approaching or moving away from the radar. Returning radio waves have a higher frequency if the particles in the cloud are moving towards the radar, and a lower frequency if the particles are moving away. This allows Doppler radars to identify severe weather phenomena. For example, a spinning vortex would have particles no longer moving towards the Doppler radar and then moving away from it a small distance.

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A radio wave is an electromagnetic wave and therefore has electric and magnetic fields oriented perpendicular to each other. This is called polarization. Dual polarization radars measure this polarization and can distinguish between heavy rain, hail, snow and sleet, as well as debris from a tornado.

When a tornado is on the ground, it throws dirt, plant matter, and other debris into the atmosphere. Because the radar is designed to detect the presence of airborne objects, it can show meteorologists where the debris is present and therefore the tornado.

“Weather Guys” Steve Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at the University of Wisconsin-Madison.


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