How does weather radar work? Learn how to predict the weather for your area with radar reflectivity imagery and learn how to spot a tornado on radar.
Meteorologists will tell you that weather radar is the best tool you can use to predict rain or snow in the next few hours – and they’re not the only ones who can use it. It’s not that hard to understand, and the web gives you access to weather radars 24/7. It can even warn you about impending thunderstorms and tornadoes.
What is Radar?
First, let’s talk basics. “Radar” is an acronym for “radio detection and ranging.” A radar set consists of a transmitter and a receiver. The transmitter sends out a pulse of energy (microwaves have replaced the original radio waves). If that pulse hits an object, a tiny fraction of that energy is reflected back. A receiver – the familiar radar dish – captures this energy. The signal is amplified and transformed into an image on a screen. By measuring the time between signal emission and return, a distance for the object can be determined.
Radar was a great asset during World War II in determining the location of enemy ships and planes, but there was one drawback: At times, the radar screen was obscured with large areas of radar returns, severely limiting its military advantage. These large areas on the radar screen were precipitation. Meteorologists immediately saw the potential for radar to be used as a weather-forecasting tool but were constrained by its military usage.
In the U.S., after a series of damaging East Coast hurricanes in 1954 and 1955, there was a greater impetus for implementing a radar network that could track these storms while they were still over the water. In 1957, the Weather Surveillance Radar (WSR-57) was developed and installed at numerous locations. In the 1970s, the more advanced WSR-74 units started to replace the older radars.
Colors of Radar Reflectivity Imagery
The early weather radars just showed areas of rain or snow as blobs of one color. Today’s radar displays have a multitude of colors. The different colors officially represent “reflectivity,” the amount of energy reflected back to the radar by the precipitation particles. The various colors represent reflectivity values in decibels (dB) of reflectivity (Z), or “dBZ.” Blue indicates the least return, then green, yellow, orange, red, and purple, and then finally whitish-gray represents the strongest return.
These color variations can reveal significant information about the precipitation. First, as you go up the scale from blue to gray, the precipitation gets heavier. With above-freezing temperatures, rain droplets show up well. Blue returns are probably evaporating before they hit the ground. As you go from green to yellow or even red, the rate of rainfall increases. If the temperature is below freezing, you’re dealing with winter precipitation. Snowflakes with a crystal structure don’t reflect the radar pulses as well, and they can show up as blue or lighter green, even if significant snow is falling. Freezing rain would show up as rain since it’s liquid until it hits the ground. Sleet pellets reflect very well and may even show up as a patch of yellow on the radar screen.
One of the greatest advances with weather radar was the ability to detect and track thunderstorms with all of their inherent dangerous elements. Radar can’t detect lightning, but the strength of a storm can be inferred from its image on the radar screen. Yellow returns are often associated with electrical activity. Storms showing as red could easily produce strong winds. Hail shows up as a strong radar return, with large hailstones even depicted as purple or whitish-gray. By watching these storms over time, you can determine the speed and direction of movement and use this to make short-range forecasts and warnings ahead of the storm.
How to Spot a Tornado on Radar
Tornadoes are the most damaging thunderstorm elements. Basic reflectivity can’t directly detect a tornado. This is where Doppler radar comes in. Doppler radar can determine how fast particles are moving toward or away from the radar site. On the Doppler velocity image, red indicates motion away from the site and green indicates motion toward it.
Strong thunderstorms tend to rotate. Sometimes, that rotation will become concentrated into a tornado on the ground. A Doppler velocity image can show this as an area where strong winds toward the radar site (green) are adjacent to strong winds away from the site (red). This is called the “tornado vortex signature” (TVS) and can be detected even before a tornado reaches the ground. This allows the National Weather Service (NWS) meteorologists to issue tornado warnings even if there haven’t been any visual reports of a tornado (officially called “radar indicated”).
With the advent of Doppler technology, the next generation of weather radars – the WSR-88D – replaced the older systems in the early 1990s and are still being used. Replacement of these radars is scheduled for the 2030s.
You Can Learn to Do It
Today, most of us have access to weather radar images. Being a great visual tool, radar depictions were quickly added to many local TV weather broadcasts and may be shown continuously during severe weather outbreaks. With the internet, you also can have continuous access to radar weather depictions.
To see your local NWS radar, click on your location on the national map. This will take you to your local NWS Forecast Office site, which includes a continuous radar image. In addition, private weather companies also provide radar displays.
So, be your own weather forecaster. If an area of precipitation shows up on your local radar, see where it is relative to your location. Then, determine in which direction and roughly at what speed the precipitation is moving. You can then determine if and when it may reach you. The same is true with thunderstorms. Just look for yellow or red returns, and make a forecast.
For more than 30 years, Ed Brotak taught thousands of college students about weather, and he’s helped many of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife, Liz.
Originally published as “Reading Weather Radar” in the July/August 2024 issue of Grit magazine and regularly vetted for accuracy.