Doppler Radar
Doppler radar is weather radar that measures the motion of precipitation as well as its location and intensity. It sends out pulses of radio waves and detects the shift in phase of the signal that returns, which shows whether targets are moving toward or away from the radar and how fast. That reveals wind patterns and rotation inside storms.
Why It Matters
Before Doppler radar, forecasters could see where rain was falling but not how the air inside a storm was moving. Velocity data lets them spot rotation that can produce a tornado and the strong straight-line winds behind a bow echo.
The U.S. network radar, the WSR-88D, uses one antenna to both transmit and receive. Each pulse lasts 1.57 millionths of a second and is repeated about 1,300 times per second, so in its usual mode the radar transmits for a little more than 7 seconds in every hour and listens for returns the rest of the time.
What a Doppler Weather Radar Measures
| Product | What it shows |
|---|---|
| Reflectivity | How much energy precipitation returns to the radar, in dBZ: where precipitation is and how heavy |
| Velocity | Motion along the radar beam: green for inbound (toward the radar) and red for outbound (away from it) |
| Dual-polarization products | Comparisons of horizontal and vertical pulses that help identify precipitation type and tornado debris. The U.S. network was upgraded in 2012. |
Source: NOAA JetStream: How Radar Works
What It Cannot See
- It measures motion only along the beam, toward or away from the radar. Wind blowing across the beam shows as little or no velocity.
- The beam climbs with distance. At 100 miles from the radar, the lowest scan angle of 0.5° is about 12,000 feet above the ground, so low-level weather far from the radar goes unseen.
- It does not measure rainfall directly. Reflectivity is returned power, and the rain rate is an estimate from it.
Frequently Asked Questions
What do red and green mean on a radar velocity image?
Green is motion toward the radar (inbound) and red is motion away from it (outbound). Bright green right next to bright red means air moving in opposite directions over a short distance, which is how rotation shows up.
How is Doppler radar different from ordinary radar?
Ordinary radar times how long a pulse takes to return, which gives a target's distance, and measures how strong the return is. Doppler radar also tracks the phase of each pulse. A moving target shifts the phase of the returned wave, and the size of that shift gives the target's speed toward or away from the radar.
How often does Doppler radar update?
It depends on the scan pattern. In active weather, the WSR-88D scans 14 elevation angles in about 4.5 minutes, and the lowest scan can update as often as every 90 seconds in some patterns.
Related Terms
NEXRAD
Next-Generation Radar, a network of 159 high-resolution Doppler weather radars operated by the National Weather Service. NEXRAD detects precipitation, wind patterns, and severe weather signatures up to 250 miles from each station.
Reflectivity
A measure of how much radar energy is reflected back by precipitation, measured in dBZ (decibels of Z). Higher values indicate heavier precipitation or larger particles. Values above 50 dBZ typically indicate heavy rain or hail.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
dBZ
dBZ, short for decibels of Z, is the unit of radar reflectivity: a logarithmic measure of how much energy precipitation returns to the radar. The scale runs from about -35 to +85 dBZ. As a guide, 20 to 40 dBZ is light precipitation, 40 to 50 is moderate, and 50 to 65 is heavy rain or some hail.
Correlation Coefficient
Correlation coefficient (CC) is a dual-polarization radar product that measures how similar the horizontal and vertical radar pulses are after they hit their targets, on a scale from 0 to 1. Uniform rain or snow gives values above 0.97. Mixed precipitation and hail give lower values, and tornado debris typically falls below 0.80.
Hook Echo
A distinctive hook-shaped radar signature on the right rear of a supercell thunderstorm, indicating rotation and high tornado probability. Hook echoes form as precipitation wraps around the mesocyclone.
MRMS
Multi-Radar Multi-Sensor, an advanced system that combines data from multiple radars, satellites, and surface observations to create a seamless, high-resolution national weather picture updated every 2 minutes.
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