Reflectivity: Intensity

LESSON 2

By the end of this lesson

You will be able to look at any reflectivity image and say how hard it is raining, whether hail is likely, and whether the picture you are looking at is telling you the truth about the ground.

Why big drops dominate everything

This one fact sets up the entire color scale. Big targets shout. Small ones whisper. And the gap between them is far wider than it sounds.

Take one raindrop and swap it for a drop twice as wide. It now sends back sixty four times as much energy. Not twice. Sixty four times. So a few fat drops can light the screen up brighter than several hundred fine ones falling in the same place, and a single hailstone can outshout an entire drizzle.

Which means the color is telling you about the size of what is up there far more than the amount.

What this actually tells you

Amount still counts. What matters is that size counts out of all proportion, so a bright red core means "there are some big things in there" rather than "an enormous amount of water is falling." Brighter does not simply mean more water. This is why hail lights up the screen so dramatically, and why a vivid return does not automatically mean flooding.

The scale

dBZ tells you how strongly something shows up on radar. It is logarithmic, so we are talking multiplication rather than simple addition, meaning the gaps are much larger than they look. Sixty dBZ is not a third stronger than forty five. It represents roughly thirty times more returned energy. You really only need to remember four numbers.

Four numbers, and everything between them is interpolation. The four are the ones worth carrying around in your head.

dBZ	What it means	Standing outside
Under 20	Drizzle, flurries, or bugs	Might not reach the ground at all
35 to 45	Real rain	Wipers on high
45 to 55	Almost certainly a thunderstorm	Pull over
Over 60	Hail very likely in the sample	Damage territory

Find the legend

Every reflectivity map has a dBZ legend somewhere around the edge of the radar. The colors themselves are not the measurement. They are just a way of displaying the number.

Find the legend on the Live Radar. Pick a color in the storm, then trace it back to the scale and see what dBZ range it represents. Different radar apps use different color tables, so do not memorize yellow = this or red = that.

Memorize the numbers. Check the legend.

One tilt, or the whole column

There are two versions of the reflectivity picture and they are not interchangeable.

Base reflectivity is one tilt of the antenna, a single tilted slice through the atmosphere. Usually the lowest one, which is the closest thing to what is happening near the ground.

Composite reflectivity looks up the entire vertical column above every point and paints the strongest value it finds anywhere in that column onto a flat map.

One storm, seen from the side, twice. Base reflectivity reports only what the lowest beam crosses, so a hail core five miles up never reaches the map and the strip under it stays green. Composite looks up the whole column and paints the strongest thing it finds anywhere in it, so that core lands on the map as though it were falling on somebody. Neither one is lying. They answer different questions.

Composite is genuinely useful for spotting the most intense cores in a big system quickly. But it will mislead you about the ground, and it is worth being precise about how, because composite is not wrong. It is accurately reporting the strongest return anywhere in the column, and that value might be from twenty five thousand feet up. A storm with a fierce mid level core that is evaporating before it lands looks identical on composite to one dumping on your roof. The measurement is honest. The question you asked it was the wrong one.

One volume scan, read two ways. The base panel is the same storm the composite panel is, minus everything that is not down low. That broad blue and green shield on the composite is the anvil and the mid level cloud, thousands of feet up and not falling on anybody.

Rule

If the question is "what is happening to people right now," use base reflectivity at the lowest tilt. Composite is great for spotting where the strongest storms are, not for ground truth.

Try it

The dBZ explorer

Drag the slider and watch what each level of the scale actually means on the ground.

Check yourself

Two samples show the same reflectivity value. One holds a few large drops, the other holds hundreds of small ones. Why do they read the same?
Size wins by a landslide. Double a drop's diameter and it returns sixty four times the energy, so a handful of big targets can match hundreds of small ones. It is exactly why hail lights up the screen.
You want to know what is actually hitting the ground right now. Which product do you use?
Base at the lowest tilt. Composite shows the strongest value anywhere in the vertical column above each point, which can be five miles up and evaporating before it lands.
A radar shows a core at 62 dBZ. What is the most defensible thing to say about it?
Hail is very likely in the sample. Reflectivity says something big is up there, not how big, and not that it survives the trip down. The scale is logarithmic too, so 62 is not twice 31.