How to Read a Weather Radar

Weather radar looks like a simple picture — splotches of green, yellow, and red moving across a map — but it's actually a measurement, not a photograph. Understanding what that measurement is, and what it isn't, is the difference between reading radar accurately and misreading it with confidence.

What radar is actually measuring

A weather radar station sends out pulses of microwave energy and listens for the fraction that bounces back off raindrops, snowflakes, or hail. The stronger the signal that returns, the bigger and more numerous the particles reflecting it. That returned-energy measurement is called reflectivity, usually expressed in a unit called dBZ (decibels relative to Z, a reference reflectivity value). Radar isn't measuring rainfall directly — it's measuring how much energy bounced back, which correlates with precipitation intensity but isn't a perfect one-to-one translation.

This matters because reflectivity can't fully distinguish between a brief, intense downpour and large hail, or between heavy rain and wet snow — all of these can produce a similar return signal. Meteorologists use other radar products (like velocity, which measures motion toward or away from the radar) and surface observations to fill in the rest of the picture; a single reflectivity image is only one piece of the puzzle.

Reading the color scale

Nearly every weather radar product uses some version of the same color progression, from essentially no return to extreme return:

The exact colors and thresholds vary slightly between products and providers, but the progression from cool, light colors to hot, saturated ones always means the same thing: increasing reflectivity, and generally increasing precipitation intensity. On RadarBug, the radar layer uses the same color scale as official National Weather Service radar products, so it lines up with what you'd see on a local TV weathercast or the Weather Service's own site.

Reading a radar loop, not just a snapshot

A single radar frame tells you where precipitation is right now. A loop — several frames played in sequence — tells you something much more useful: where it's going and whether it's getting stronger or weaker. A few things to watch for in a loop:

• Storm motion — the direction a cell or line is traveling tells you whether it's headed toward you, and roughly how much time you have before it arrives.
• Growth vs. decay — a cell that's expanding and intensifying in color from one frame to the next is strengthening; one that's shrinking and fading toward green is weakening.
• New development — small cells popping up ahead of an existing line (sometimes called "discrete" cells) can become severe faster than the parent storm they're feeding off of.
• Hook-shaped or curved segments on the trailing edge of a storm can indicate rotation, which is one visual cue (among several meteorologists use) associated with tornado potential — this is why a Tornado Warning is sometimes issued based on radar signatures alone, even before anyone reports seeing a funnel cloud.

Radar's blind spots

Radar is extremely useful, but it has real, physical limitations that are worth understanding rather than discovering the hard way:

• The beam curves upward with distance. Radar doesn't look flat along the ground — it's aimed slightly upward and the Earth curves away beneath it, so the farther you are from the radar site, the higher above the ground the beam is actually sampling. A storm 100+ miles from the nearest radar can be scanned several thousand feet above the surface, missing what's actually happening at ground level (rain that evaporates before reaching the ground, called virga, is a common result).
• Ground clutter and anomalous propagation can make non-weather returns (terrain, buildings, even dense insect or bird activity) show up as speckled or ring-shaped echoes that look like light precipitation but aren't.
• Radar doesn't show lightning, wind gusts, or temperature directly — it shows precipitation. A storm can be producing dangerous lightning or damaging straight-line winds with a radar signature that doesn't look especially dramatic.
• It doesn't replace an official warning. Radar is one input among several (surface observations, weather balloon data, spotter reports, satellite imagery) that meteorologists use to issue watches and warnings. A storm can look modest on radar and still warrant a warning based on other information, or vice versa.