Radar School › Lesson 1
LESSON 1Open the radar
Tap the Live Radar button to open the in-lesson radar. It has the standard controls and layers other radar apps have. You will use it throughout the course, so take a minute to get comfortable with it.
Find these three controls:
- Products. Open the menu and switch radar products. You do not need to understand them yet.
- Radar site. Find the selected radar and its dish on the map. Radar always has a point of view, and later you will learn why that matters.
- Play / pause. Run the loop, stop it on a frame, then start it again. Radar is meant to be watched in motion, not read as a single picture.
Lesson 9 goes deeper into how radar apps work.
Each lesson will give you something new to look for in the live radar. Look for:
Try it now!
Two Pictures
A weather radar makes two completely different pictures of the same storm. By the end of this lesson you will know which picture answers which question, and which one to open when what you want to know is whether a storm is rotating.
One radar produces two completely different pictures, and they answer different questions. Most people only ever meet the first one.
Everything about how hard it is raining lives in the left picture. Everything about which way the air is moving lives in the right one, and it is not discernible from the left one at any level of skill. They are two different measurements that happen to arrive in the same radar scan.
The two products
Think of product = a particular processed view of the radar's measurements. The two main ones are:
Reflectivity measures where the precipitation is and how strongly it is returning energy. More energy generally means larger or more numerous targets, which often means heavier precipitation. This is the familiar green, yellow, and red radar picture.
Velocity measures which way the air is moving relative to the radar. It uses the Doppler shift to measure motion toward or away from the radar.
You already know this effect. A siren coming toward you sounds higher than the same siren going away, and it drops the instant it passes. Christian Doppler explained why in 1842, which is why the thing on the evening news is called Doppler radar. It is listening for exactly that shift, at a pitch no ear can hear and a precision no ear could manage.
Same radar, same scan, completely different information.
What the radar is actually doing
A weather radar does one thing, over and over. It fires a very short burst of microwave energy into the sky, then shuts up and listens. Whatever the burst hits, raindrops, hail, bugs, a hillside, scatters a tiny fraction of the energy back. The radar catches that echo and times the round trip, which gives it a distance.
Then it turns a few degrees and does it again. Once it has swept a full circle it tilts up a notch and sweeps again. A dozen or so tilts stacked together make a volume scan, and the whole three dimensional picture refreshes every five minutes or so.
The scale involved is genuinely absurd. The outgoing pulse is around a megawatt. What comes back is around a nanowatt, a billionth of a watt. The radar is shouting at the sky and straining to hear a reply a quadrillion times fainter.
The radar only knows what is along the beam, only at the instant the beam is there, and only at whatever height the beam happens to be at that distance. Nearly every quirk and every lie you will meet in the When Radar Lies lesson traces back to those three limits.
There is no such thing as "the radar"
There is no single national weather radar. There are 159 individual towers covering the United States, Puerto Rico and Guam, run jointly by the National Weather Service, the FAA and the Air Force, and every one of them is a dish in a dome doing exactly what the last section described.
What most weather apps show you by default is a mosaic: a collage stitched together from many of those towers into one clean regional or national map. That is genuinely useful for finding and following storms. But once you want to study one storm closely, switch to a single radar near it and do the detailed reading there. Everything else in this course assumes you have made that switch.
Explain further
When you study one storm, you want all of your evidence to come from the same point of view. That matters especially for three reasons.
- Velocity only makes sense relative to a specific radar. "Toward" and "away" literally mean toward or away from that tower. Move to another radar and the colors can change dramatically even though the storm has not changed.
- The beam gets higher as it travels away from the radar. One tower may be sampling a storm near the ground while another is looking several thousand feet higher into the same storm. If a mosaic combines them, you lose that context.
- Products belong together. Reflectivity, velocity, correlation coefficient, different tilts, and the animation from one radar are all measurements of the storm from the same location and volume scan. You can compare them directly.
Reflectivity stitches together tolerably, because how hard it is raining in a place does not depend on who is looking at it. Velocity does not stitch, because velocity means toward or away from that particular dish. Ask a mosaic which way the air is moving and the question has no single answer, since every tower in it would answer differently about the same air. You can see this in the Live radar: zoomed out to the national view, there is no velocity layer on offer.
So the moment you want velocity, you are on one site whether you meant to be or not.
Every lesson from here focuses on reflectivity, velocity, or both. The next three lessons stay inside reflectivity, because there is more in that one picture than most people ever get out of it. The Green and Red lesson is where velocity opens up, and it is where reading radar starts to feel like a superpower rather than a chore.
The habit this course is building
Radar does not hand you conclusions. It hands you evidence, one product at a time, and reading it well is mostly the discipline of not skipping ahead. Three steps, and they repeat on every storm you will ever look at.
See. Something catches your eye. A shape, an edge, a color where it does not belong. No conclusion yet, just a thing you noticed.
Suspect. Say what it might mean, and keep it provisional. "That southwest side looks like it is wrapping" is a suspicion. "Tornado" is not a suspicion, it is a leap.
Verify. Go find a second product, or a second frame, that supports the suspicion or kills it.
One clue should always make you go and check another. What separates people who read radar from people who look at it is the third step, and specifically the willingness to let the second product kill the idea.
Which picture?
Eight questions. For each one, decide which of the two radar images could actually answer it.
A storm looks impressive on the reflectivity picture. What can that picture tell you about whether it is rotating?
Nothing at all. Reflectivity measures how much energy came back, which is about the size of what is up there. Rotation is motion, and motion lives in the other picture entirely.
Roughly how often does a radar rebuild its full three dimensional picture of the sky?
About five minutes for a full volume scan. That is why looping matters. You are watching a flipbook, and each page took the radar a few minutes to draw.
The radar transmits around a megawatt and receives around a nanowatt back. What does that tell you about the measurement?
It is straining. The return is about a quadrillion times fainter than what went out, which is why terrain, bugs, distance and heavy rain in the way all cause real problems later on.