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In this article 8 sections
The Basics: Weather radar measures "Reflectivity," which is how much energy bounces off raindrops and returns to the sensor. This is measured in units called dBZ (decibels of Z). In simple terms:
🟢 Green (20-30 dBZ): Light Rain.
🟡 Yellow (35-45 dBZ): Moderate Rain.
🔴 Red (50-60 dBZ): Heavy Rain / Thunderstorms.
🟣 Purple/Pink (65+ dBZ): Hail, Debris, or Extreme Rain.
- 🌪️ Hook Shape: Possible Tornado (Rotation).
- 🏹 Bow Shape: Damaging Straight-Line Winds.
- ❄️ Blue/White: Snow (in winter mode).
- ❌ Cone of Silence: The empty hole directly above the radar station.
How Radar Actually Works: The "Echo" Concept
We see these colorful blobs on TV or on our phones every day, but have you ever wondered how they get there? It isn't a satellite picture; it is an echo.
RADAR stands for RAdio Detection And Ranging. Think of it like a bat using echolocation. The radar station sits on a tall tower and sends out a pulse of microwave energy. When this pulse hits an object, like a raindrop, a snowflake, or even a bird, it bounces back.
The radar measures two things: how long it took for the signal to return (which tells us the distance) and how strong the return signal is (which tells us the intensity). Once you can read these echoes, you can tell in a minute whether to cover the patio furniture or carry on with the barbecue. Our weather education pages explain more of the basics.
🎨 Decoding the Colors (The dBZ Scale)
The "Reflectivity" mode is what you see 99% of the time on apps like Weather Underground. Meteorologists use a color scale based on the intensity of the return signal.
The Color Spectrum Explained:
Green (Light Precipitation): This is your standard drizzle. It is usually safe to drive in, though the roads might be slick. Sometimes, high humidity or bugs can even show up as faint green fuzz.
Yellow/Orange (Moderate): Now we are talking about real rain. This is steady rainfall. You will need windshield wipers on medium or high.
Red (Heavy): Usually the core of a thunderstorm. Expect heavy downpours and lightning. If you see red approaching, head indoors. The very highest values (pink, purple or white on many apps) can mean hail: the National Weather Service describes 50 to 65 dBZ as heavy precipitation or some hail (NWS JetStream: reflectivity).
One honest limit: the colors show how much energy bounces back, not how much rain reaches the ground. Radar does not measure rainfall directly. Colors also differ a little from app to app, so check the legend of yours.
Dangerous Shapes: When to Seek Shelter
Colors tell you how hard it is raining, but shapes tell you how the storm is behaving. Severe weather leaves distinct fingerprints on the screen.
This is the most famous shape. If a storm looks like a kidney bean with a small hook curling off one side, the storm is probably rotating. The National Weather Service calls a hook echo a good sign that a storm has rotation, and tornadoes in such storms are often found at the tip of the hook. It is a warning sign, not proof: plenty of hooks never produce a tornado, and some tornadoes show no clear hook at all.
Sometimes a line of storms bulges out in the middle, like an archer's bow. Strong winds behind the storm are pushing it forward. A bow echo points to damaging straight-line winds. A widespread, long-lived windstorm of this kind is called a derecho.
This isn't a single shape, but a behavior. If you see red and yellow blobs moving over the same spot repeatedly, like train cars on a track, this leads to flash flooding. This phenomenon is often associated with the moisture feed from an atmospheric river.
Long lines of storms often form along a cold front. If you want to know why, see how weather fronts work. And if the power or the internet is out, here is how to tell a storm is coming without a forecast.
Velocity Mode: Seeing the Wind
Most public radar apps focus on precipitation, but advanced users toggle to "Velocity" mode. This uses the Doppler effect to see which way the raindrops are moving.
In this mode, the colors change completely:
- Green: Wind moving TOWARD the radar station.
- Red: Wind moving AWAY from the radar station.
⚠️ The Couplet
If you see bright green right next to bright red, pay attention. It means wind is blowing toward and away from the radar very close together, which points to rotation. Forecasters call a strong version a tornado vortex signature. Radar does not see the tornado itself: it sees the rotation around it (NWS JetStream: velocity). That is why warnings also lean on storm spotters and on debris showing up on radar, and why this view is so useful at night, when nobody can see the sky.
Winter Radar: The "Messy" Mix
Winter radar is harder to read. Snow reflects energy differently than rain because snowflakes are fluffy and scatter the beam. This is why light snow often looks like faint blue or grainy white.
On many apps, pink is the color to watch in winter. It usually stands for a wintry mix: sleet or freezing rain. Freezing rain is hard on power lines and dangerous to drive in. If you see a change from blue (snow) to pink (ice) to green (rain), you are looking at the melting line. Review your winter storm safety plans if pink is heading your way. Color schemes differ per app, so check the legend.
Also, beware of the bright bands in winter. Sometimes melting snow looks like "heavy rain" (bright yellow) on radar because wet snowflakes reflect like crazy, even if the snowfall intensity isn't actually that high. This is known as "Bright Banding."
🌍 Radar Ghosts: What Is Not Real?
The radar sees everything, but not everything is weather. Sometimes the beam hits other things, especially on clear days. If you have a Zivron WiFi weather station reporting clear skies, but the radar shows blue blobs, you might be seeing "Ground Clutter."
| Phenomenon | What it looks like | What it actually is |
|---|---|---|
| Ground Clutter | Stationary grainy blobs near the center. | Buildings, trees, or hills reflecting the beam. |
| Sun Spikes | A long, thin line pointing at the sunrise/set. | The radar picking up radio energy from the sun when the dish points at sunrise or sunset. |
| Roost Rings | Expanding donut shapes at dawn. | Thousands of birds or bats taking off at once. |
| Chaff | Long, fuzzy bands. | Military aircraft releasing radar-jamming fibers. |
📚 Radar & Weather Geek Essentials
Radar tells you what is coming. A sensor in your own yard tells you what has arrived. If you are thinking about one, start with our comparison of the 5 best home weather stations.
Video Explanation
Seeing is believing. This video breaks down the different radar modes and shows real-life examples of storm signatures.
Frequently Asked Questions
Why is there a circle of "clear" weather around the radar?
This is called the Cone of Silence. The radar dish points outward and upward, but it cannot look directly up at 90 degrees. Therefore, storms directly overhead often disappear from the screen, even though it might be pouring rain at the radar site.
What does "Composite" vs. "Base" reflectivity mean?
Base Reflectivity shows only the lowest slice of the storm, closest to the ground. Composite Reflectivity takes the strongest return from every scan height and merges them into one image. Composite is better for the total strength of a storm. Base is better for small features near the ground, such as a hook echo, which composite often hides.
Can radar detect tornadoes?
Radar detects the rotation that leads to tornadoes (using Velocity mode) and the debris tossed by a tornado (using Correlation Coefficient). However, a "Tornado Warning" based on radar means rotation is detected, but it may not be touching the ground yet. Always take cover regardless.
📝 The Radar Watcher's Cheat Sheet
Keep this handy for the next severe weather outbreak:
- ✅ Green/Yellow: Standard rain. Drive carefully.
- ✅ Red/Purple: Heavy storm core. Hail is possible. Head indoors.
- ✅ Tight Hook Shape: Rotation, possible tornado. Check for a warning and be ready to take shelter.
- ✅ Line Bowing Out: High winds approaching.
- ✅ Stationary Blobs: Ground clutter or buildings (False Alarm).
Last updated: October 9, 2026. The radar explanations were checked against the National Weather Service's JetStream lessons on reflectivity and velocity.
