Radar School › Lesson 9

Your Radar App

You have spent eight lessons finding these controls in the live radar built into this course. The gap left is not knowing what a hook echo is, it is finding one on your own phone at nine at night, in an app with a different layout and nobody guiding your tap. This lesson closes that one.

The good news is that radar software fragments much less than it looks like it does. Every app, including the one you have been using, is reading the same government data feed, so everything that actually matters is identical everywhere. What differs between apps is where the buttons live and what they are called, and that part is not worth memorizing for any one program.

The four controls

Strip away the styling and every radar application ever built, this course's included, is a map with four controls on it. You have already used all four: switching products, picking a site, changing tilt, and running the loop. Here they are again, named plainly enough to recognize in whatever you open next.

Find those four in whatever you are holding and you can operate it. Find them in a different app next year and you can operate that too, because you already have the habit and not just the theory. Everything else on screen is decoration or convenience.

One radar product at a time

A radar app is a stack of layers on a map. Warnings, storm reports, lightning, roads, county lines: all of those can be switched on together, and most of them stack in any combination.

The radar itself is the exception. You get exactly one product drawn at a time, which you have been doing every time you switched from reflectivity to velocity in the live radar. It trips up almost everyone at first: the storm looks like it vanished and was replaced with something unrecognizable. It did not vanish. You changed the question.

The answer is a side by side view. Many higher end radar apps (usually ones on a desktop with a bigger screen) offer it under some name like dual pane or multi pane: reflectivity on the left, velocity on the right, same storm, same moment. If your app can do it, that one feature will teach you more than any other setting. The live radar in this course does not have one; flipping quickly back and forth between two products on the same scan is the next best thing.

Read the actual number

Every product you have learned is colors standing in for a number: this shade of green is around 35 dBZ, this shade of red is outbound at some speed. Reading colors by eye is a real skill, and it is the one this course has been teaching, but it is also an estimate. Most radar apps carry a tool built for exactly this: skipping the estimate and reading the actual measurement under one point, rather than guessing from the shade nearest to it.

The live radar in this course has one, called the probe here and an inspector or a crosshair elsewhere. Turn it on, tap the picture, and it answers with a number instead of a color: dBZ, wind speed, or a bare CC value, whichever product is currently up.

Try it live

Reading the menu

You have been looking at product names and Tilt 1 through 4 the whole time you have used this course's live radar. Somewhere in your own app is a list that looks like this, with no explanation attached.

N0BN0QN0UN0GNABN1CN0XNCRN0S

It is not random. Every code reads left to right and says three things.

So N0B is the normal radar family, the lowest tilt, super resolution reflectivity. That is the picture you have been looking at for eight lessons. And N1C is the normal family, the 1.3 degree tilt, correlation coefficient, which is the uniformity check from When Radar Lies, one tilt up.

Three radar products carry the course

There are a dozen products in that menu and you have been taught three. That was not an oversight. Reflectivity, velocity and correlation coefficient answer the three questions that actually decide what you do, and everything else in the list refines an answer rather than giving you a new one.

Still, you will see the others, so here is what each is asking, once, so the menu stops being intimidating.

• ZDR, differential reflectivity. Compares the horizontal pulse with the vertical one to ask whether targets are wider than they are tall. Big raindrops flatten as they fall, so they read wide. Hail tumbles, so it reads round. It is genuinely useful and genuinely easy to misread.
• KDP, specific differential phase. Best single measure of how much liquid water is actually falling, which makes it a rainfall and flooding tool more than a severe weather one.
• Spectrum width. How much the motion varies inside one sample volume. High values mean turbulent, ragged flow. Occasionally a useful supporting clue, rarely a deciding one.
• Echo tops. How high the echo reaches. The fastest way to spot which cell in a crowd is the most vigorous, because tall means a strong updraft is holding it up.
• VIL, vertically integrated liquid. Adds up the water in the whole column. You will see it sold as a hail detector. Treat it the way Intensity taught you to treat 60 dBZ: it says something big is up there, not how big, and not whether it reaches the ground.

None of those is worth learning before the three are automatic.

The layers that are not radar

Everything above this point is a radar product: a measurement made by a dish. The rest of the switches in your app are not radar at all, but instead layered over or under the radar picture from a source that never touched a dish. You have already tried one, in Specialists: ground temperature.

These each earn their place, because radar is one witness.

• Lightning. Shows where storms are actively producing lightning. A useful cross-check that an echo is truly convective.
• Warnings and watches. Shows what the National Weather Service is concerned about. Compare the warning area with what you see on radar.
• Surface observations. Adds real ground measurements like wind, temperature, and dew point. Useful context for what the radar is showing above.
• Satellite. Shows the clouds instead of the precipitation. Useful for seeing storms developing before radar has much to show.

Radar observation versus models

Somewhere in the menu, probably near the bottom, you will find HRRR or something labeled "future radar." It looks exactly like radar. Same colors, same shapes, same map, and it will happily show you a squall line at four o'clock this afternoon.

It is not radar. It is a numerical weather model, which means a simulation of what the atmosphere might do, rendered in radar's clothing.

A tilt is an angle, not an altitude

The number beside your tilt control, usually starting at 0.5 degrees, is how far the dish is tipped up. It is not a height. Because the beam leaves at an angle and the ground curves away underneath it, the same 0.5 degree scan is looking at completely different altitudes depending on how far away you are looking.

This is the beam climb problem from When Radar Lies, and here is the practical version of it. A rotation signature 120 miles away is a signature two miles above the ground.

Why some things cost money

The radar produces two kinds of output, and knowing which is which explains the entire pricing landscape without anybody having to describe an app to you.

Most apps serve Level III because it is small and the useful summaries are already built. Level II is where all the tilts live, and handling it costs a company real bandwidth and computing, which is why "all tilts" and "archive" tend to sit behind a subscription.

Worth knowing: the underlying data is free. Every scan since 1991 is public and sits on a public server at no charge. What a subscription buys is somebody doing the decoding for you.

The trap in the colors

This course taught you numbers. Over 60 dBZ means hail is likely. In velocity, green is toward the radar, red is away. Those are facts about the data and they will hold anywhere.

The colors are not facts. Every app ships its own palette and many let you load your own, so the shade sitting at 60 dBZ in one program is somewhere else entirely in the next. Learn "purple means hail" and you will eventually open an app where a hail core renders deep red.

Which radar site?

Sites have four letter names like KGRR or KDTX. The first letter is a region code, not part of the place name, and the radar is frequently nowhere near the city it is named for. Detroit's radar sits about thirty miles out of town.

Start on the mosaic, then switch to a single tower to study anything. This is how you pick the tower.

Nearest is usually right, because nearest means lowest, and lowest means closest to the ground where people are. Three exceptions are worth carrying.

• Directly overhead is a blind spot. The radar cannot look straight up, so a storm passing within about twenty miles has its upper half unsampled. It will appear to weaken as it arrives and strengthen as it leaves. It did not do either.
• Rotation shows best from the side. Velocity only measures motion straight toward or away from the radar, so a circulation viewed from an unhelpful angle can be faint on one radar and obvious on its neighbor. If something looks marginal, check the other site.
• Terrain gets in the way. A radar in hilly country has directions it cannot see into at low tilts, because a ridge is in the beam. The blocked sector is always blocked, so a storm that seems to weaken every time it crosses one particular bearing is worth suspecting.

Where to go from here?

You do not need a fancy radar app to use what you learned here. Start with the tools you already have, then add more only when you know what you want from them.

A computer gives you room to compare products side by side, which is one of the best ways to study a storm. A phone gives you speed and access when something is happening.

The important part is not the app or device it's on. It is knowing what you are looking at, what the radar can tell you, and when to check another product or source.