Radar School › Lesson 5
LESSON 5Velocity: Green and Red
You will be able to find rotation in a storm yourself, instead of waiting for a warning polygon to tell you it is there.
What velocity actually measures
At every spot on the map, velocity reports one number: how fast the air is moving toward or away from that radar. Motion straight across the radar beam contributes nothing, so even a 100-knot (115 mph) sideways wind can read as zero.
That also means velocity is always relative to one particular radar. Another radar viewing the same wind from a different angle can measure a different velocity. That is why velocity is best read from a single radar, not a mosaic.
Stand on a corner with a stopwatch and measure one thing only: how quickly a car is getting closer to you. A car driving straight at you closes the gap fast, and your number is large. A car crossing the intersection in front of you never gets any closer, so your number is zero, even though it is doing forty. Your measurement is not wrong. It is answering a narrower question than "how fast is that car going."
The color convention
- Green (cool colors) is inbound. Coming toward the radar.
- Red (warm colors) is outbound. Going away.
- Gray is zero. Either stationary, or crossing the beam.
- Purple, often marked RF, is no data at all. The When Radar Lies lesson covers why.
Why one wind paints two colors
One steady wind, blowing the same direction across the whole radar range, does not paint one color. It paints two.
Pull up a real scan and find that same split for yourself: green on one side of the radar, red on the other, one wind behind both.
A change in velocity color does not always mean the wind itself changed. Sometimes only the angle between the wind and the radar changed. Reading velocity is largely the skill of subtracting out that geometry.
Brightness is speed
Color is direction. Brightness is speed. Dark green and dark red sit near zero, and the colors brighten as the speed climbs, so a small patch of vivid yellow-orange outranks a large patch of dull red. Check the color bar before you read anything: the top of the scale is not the same number in every app.
Which way is the wind blowing
The rule: wind blows from the green side toward the red side. Because wind is named for where it comes from, name the wind by the green side.
Read the wind
Here is a real velocity field at a random angle. Call the wind direction.
What you are actually hunting for
Big smooth fields of green and red are just wind. Interesting, occasionally useful, not urgent.
What matters is a small tight spot where intense green sits directly against intense red. That means two pockets of air right next to each other moving in opposite directions, which means some local movement is happening. That pairing is called a couplet, a matched pair of velocity bull's-eyes, one strongly inbound and one strongly outbound, sitting next to each other. It might be the most important word in the whole course.
The test
What a couplet means depends completely on how the pair is arranged relative to the radar.
Side by side means spinning. One behind the other means spreading.
Draw a line from the radar through the colors. That line is called a radial. If the two colors sit side by side across that line, you are looking at rotation. If they sit one behind the other along that line, the air is either spreading apart or coming together.
Spreading apart can be a clue to a downburst. Coming together is convergence, which can help support rising air.
Look again at the rotation diagram. The parts of the circulation moving across the beam are just as fast as the parts moving along it, but the radar barely registers them. A circulation is always at least as strong as it appears, and usually stronger. That understatement gets worse with distance, because the sample volume grows.
The four cases, named
These are all four, at four different angles from the radar, with the radial drawn through each one. Cover the captions and work each one out before you read it. Then uncover them.
How to reason through it, not memorize it
Draw a line from the radar through the middle of the pair. Again, that is the radial.
Then ask two questions:
- Are the colors side by side across the radial, or stacked along it?
Side by side = rotation
Stacked = convergence or divergence - Where is the inbound color?
If they are side by side: inbound on the left means counterclockwise rotation. Inbound on the right means clockwise rotation.
If they are stacked: inbound closer to the radar means divergence. Inbound farther from the radar means convergence.
That is the whole test: shape first, color position second.
When the couplet does not look like a couplet
So far the rotation has been easy to spot: green beside red. Real storms do not always give you that picture. Everything up to here quietly assumed the storm was holding still. Motion spent a whole lesson on why that is never true.
The thing that matters is not whether one side is green and the other is red. It is the difference between neighboring velocities.
Say a storm is racing straight away from the radar at 55 knots. Inside it, a circulation is turning: one side of the spin is moving 20 knots toward the radar, the other side 20 knots away.
The radar cannot separate the two. It reports the sum.
| The side of the spin | Carried by the storm | Its own spin | What the radar reports |
|---|---|---|---|
| turning toward the radar | 55 away | 20 toward | 35 away |
| turning away from the radar | 55 away | 20 away | 75 away |
Both sides are moving away, so both sides paint red, and a reader hunting for a color change finds nothing. But a jump from 35 away to 75 away is still a strong rotational signature.
The circulation is entirely there. There is a 40-knot velocity difference across it, exactly as there would be if the storm were sitting still. It is just wearing one color.
Slide the storm's own speed
One rotating storm, and one control: how fast the whole thing is running away from the radar. The rotation never changes. Watch the two numbers move, watch the colors go, and watch the gap between them refuse to budge.
Storm relative velocity
The fix is a second velocity product, usually labeled SRM or SRV for storm relative velocity. It subtracts the estimated storm motion from base velocity, making motion relative to the storm easier to see. The storm's 55-knot motion pushed both sides outbound, 35 away and 75 away. Subtract it and they become 20 toward and 20 away: the same couplet, back in the colors you already know how to read.
SRV does not find rotation that was not already there. It makes existing velocity differences easier to see.
SRV removes part of the storm's motion, so its numbers can make strong winds look weaker. Switch back to base velocity when you want to judge wind speed.
Couplet trainer
Eight couplets at random angles. Draw the line from the radar in your head and call it.
A steady wind blows from the west at 45 knots across the entire radar range. What does the velocity display show?
Green one side, red the other, zero across the middle. The wind never changed. The viewing angle did. The radar only measures the part of the motion aimed at it or away from it.
You find a tight couplet. Drawing a line from the radar through it, the green and red sit on either side of that line. What is happening?
Rotation. Side by side across the line of sight means spinning. If the beam had met one and then the other along that line, it would be air spreading out or squeezing together instead.
On a velocity display, the green half sits over the southern part of the screen and the red half over the northern part. What is the wind doing?
Green marks where the air is coming from, so this is a south wind, and a south wind travels north. The name and the direction of travel are opposites, which is the single most common source of confusion in reading these images.