The US radar network is called NEXRAD: 160 or so Doppler radar sites covering the country, each one sweeping a rotating beam of microwave energy across the sky. When that beam hits something, raindrops, hail, snow, insects, a flock of birds, a fraction of the energy scatters back. The radar times the return to get distance and measures its strength to estimate what's out there.
What the colors mean
The standard product is reflectivity, measured in dBZ, and it reflects how much energy came back. Bigger and denser targets return more.
The Doppler part measures motion: whether targets are moving toward or away from the radar. That's how forecasters spot rotation inside a storm, which is the basis of most tornado warnings.
The limitations nobody mentions
- The beam rises. Radar tilts slightly upward and the earth curves away beneath it. At 100 miles out, the lowest beam is already thousands of feet up, so it can miss shallow rain entirely.
- Virga fools it. Rain that evaporates before reaching the ground still shows up. Radar says it's raining; your driveway disagrees.
- Terrain blocks it. Mountains create permanent shadows where nothing is seen.
- It's not instant. A full volume scan takes several minutes, so what you're watching is always a few minutes old.
- Non-weather returns are common. Bird migrations, insect swarms, wind farms, and even smoke plumes all echo.
When radar shows green over your town but the nearest station reports dry, both can be right, the beam may be sampling rain thousands of feet up that never reaches the surface. A ground sensor reporting rain is the stronger evidence about the ground.
Who pays for it
NEXRAD is operated jointly by the National Weather Service, the FAA, and the Department of Defense. The data is public and free. Which means every weather app on your phone, including the one charging you $4.99 a month for "premium radar," is reselling you a picture your taxes already bought.