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Why are there red and green radar images in my Twitter feed during tornado outbreaks?

Why are there red and green radar images in my Twitter feed during tornado outbreaks?
A view of a rain-wrapped tornado looking south from Hazel Green High School is seen as multiple tornadoes raked across Hazel Green and northern Madison County late afternoon Monday, April 28, 2014, in Hazel Green, Ala. (AP Photo/AL.com, Eric Schultz)

When there is a major tornado outbreak as there have been in the past two days, if you follow any friends (or meteorologists) in the Midwest or Southeast, your social media feeds might be filled with radar shots of the places in danger.

While some of those views might show the classic "reflectivity" scan, which is the traditional radar page we show in Seattle all the time that shows the greens, yellows, reds -- and into the purples and beyond for really severe weather back East -- for intensity of precipitation, there's another view you might see a lot of online that shows just two main colors: red and green.

This is called a "velocity scan" and is a measurement of wind speed and direction. The radar beam uses -- what else? -- the Doppler effect to gauge whether particles in the air are moving toward or away from the radar site.

The radar then displays particles that are moving away from the site in green and coming toward the site in red. On a routine day, a radar image will look like this:


(Photo courtesy: RadarScope)

Here you can see the reds on the north side of the Seattle-area radar site at Camano Island, and the greens on the south side. This is just indicating a generally north wind -- particles coming toward the radar from the north and moving away from the radar to the south.

But this radar feature can be critical in spotting tornadoes, because a tornado's rotating winds will clearly show up with a very obvious swirl of green and red close together, as the winds on one side of the tornado come toward the radar, and then blow away from the radar on the other side. Here are some examples from this week...

This first one is from the Birmingham, Alabama radar Monday evening -- this actually shows two tornadoes on the ground! Note the green and red swirl southwest of Tuscaloosa and another one due west of Birmingham. (The red boxes indicate active Tornado Warnings; yellow are Severe Thunderstorm Warnings.)


(Photo courtesy Benjamin Jurkovich and JWSevereWeather Organization. Radar graphic courtesy RadarScope App)

Here is another radar shot -- this from a tornado that struck near Brandon, Miss. Monday evening. Can you spot it on the radar? (The blank circle is from the radar itself as there is a always a "cone of silence" in the immediate area surrounding the radar beam)


(Photo courtesy Mark Weinberg, WDRB-TV, Louisville, KY)

Sometimes in severe tornadoes, the radar can even pick up the debris being tossed about by the twister.

It wasn't all that long ago that tornadoes were mainly discovered only when sighted by spotters or witnesses. Finding tornadoes on radar was possible, but difficult and it would miss many of them.

But now with the advancements in Doppler Radar, forecasters can better spot tornadoes even before they actually form, or know with high likelihood that a tornado has formed and is on the ground -- to the point it is very rare a major tornado sneaks up on anyone anymore.