Radar School › Lesson 3
LESSON 3Reflectivity: Shape
You will be able to name the organization type of almost any storm on screen, and say what that type is capable of before you read a single number off it.
Intensity was about how bright. This lesson is about what shape, and shape carries far more information than intensity does. A 55 dBZ core tells you it is raining hard somewhere. The shape it sits inside tells you what the storm is capable of.
Four basic shapes cover most of what you will see: scattered showers, multicell clusters, squall lines, and isolated cells. Learn the shape first, then the name. Once your eye knows the pattern, identification gets fast.
The fastest read available
Before you name anything, look at how quickly the color climbs from the outer edge of a cell to its core. This is the reflectivity gradient.
Both cells can peak at the same value. The one with the sharper gradient gets there in a fraction of the distance.
A sharp edge is often a sign of a strong updraft keeping the storm's heaviest precipitation packed tightly together. You can see that without reading a single number.
It is a clue, not a law. A small developing cell or the edge of an echo the radar is clipping can look sharp too. Compare the edges within a storm, and compare storms with each other.
Scan a field of storms and the cells with knife-edge gradients are worth a closer look.
One of the most useful distinctions
Isolated. A storm standing by itself with clear air around it has a better chance of maintaining clean, undisturbed inflow. In a favorable environment, that can help it stay organized or become a supercell.
Crowded. Storms crammed into a line are competing for the same air. Lines are especially good at producing damaging straight-line wind.
The practical takeaway: an isolated cell, especially one breaking away from a line, deserves a closer look. It may have cleaner inflow and more room to organize. But isolation is only a clue. Reflectivity can raise the suspicion; velocity is where you check for rotation.
Which one do you watch?
You do not need to study every storm on the screen equally. Start with three questions.
- Is it organized? A coherent line or isolated cell gets more attention than scattered blobs.
- Does it have a sharp edge? A tight reflectivity gradient can point to a stronger updraft.
- Does it have room? An isolated storm with clean air around it may have a better opportunity to stay organized.
None of those proves a storm is severe. Together, they tell you where to look next. That is the skill: not diagnosing a storm from reflectivity alone, but scanning twenty storms and knowing which two or three deserve a closer look.
Straight-line wind from a bow echo can produce a huge swath of damage and is sometimes mistaken for tornado damage. Straight-line wind tends to push damage generally in the same direction, while tornado damage can show converging or rapidly changing directions. Damage surveys use several clues, not debris direction alone.
Name that storm
Ten real storms, one at a time. Call the shape. The easy ones come first.
Two cells have identical peak reflectivity. One climbs from green to red across a very short distance, the other fades in gradually. What does the sharp one tell you?
Stronger updraft. A powerful updraft holds heavy precipitation in a tight column instead of letting it spread out and settle, so the edge stays sharp. It is the fastest strength read available and it needs no numbers.
Which storm is most likely to produce a tornado?
The isolated one. A storm by itself has a better chance of keeping clean, undisturbed inflow, which can help it stay organized or become a supercell. Storms crammed into a line compete for the same air, and lines are especially good at producing damaging straight-line wind.
A squall line develops a bulge that pushes out ahead of the rest of the line. What is it, and what does it tell you?
A bow echo. The bulge is the line being shoved forward by descending air, and the apex of the bow is the stretch of ground you would least want to be under.