The other evening I was checking out the NWS page for Tucson, and I clicked on the local Radar image. This is what I saw:
Tucson Base Reflectivity
At first glance, it would appear that it was raining outside in the areas indicated in blue. But, having spent the evening outside coaching a softball game, and having checked the skies before heading in for the evening, I knew that, if anything, the only thing in the sky was some high clouds. So, why would the radar indicate that it was raining? I decided that it was time to get a better understanding of what Base Reflectivity was all about.
Turning to Google, I found a link to the National Weather Service page on Base Reflectivity. I started reading the FAQs on weather radar, and it became very apparent that there is more than meets the eye when reading weather radar output.
To begin, we need to understand how weather radar works. Basically, the Next Generation Radar (NEXRAD) obtains weather information (precipitation and wind) by measuring returned energy. The radar sends out a burst of energy (green), and if the energy strikes an object like rain drops, bugs, birds, etc., the energy is scattered in all directions (blue). A small fraction of that energy gets directed back to the radar. 
The radar has a listening period, in which it collects and analyzes the signals that it receives. The whole process to analyze the signal is super fast, and occurs around 1300 times per second! In an average hour, the radar spends about 7 minutes sending signals, and 53 minutes listening for them. Based on some geeky physics stuff, the analysis can tell the “phase shift” of the signals it receives, which lets it know in what direction, and how fast the object it got bounced off of is going. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to “see” the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.
Base Reflectivity, which is what’s on the map above, is a display of echo intensity (reflectivity) measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). “Reflectivity” is the amount of transmitted power returned to the radar receiver. Base Reflectivity images are available at several different elevation angles (tilts) of the antenna and are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential.
When you look at the Base Reflectivity map, you’ll see various colors on it, and one of the scales that you see to the left of this text. If the radar is operating in “clear aid” mode, then the values range from -28 to +28 dBZ. If the radar is operating in “precipitation mode,” then the values range from 5 to 75 dBZ. Turns out, the map I was viewing was operating in clear air mode, which was something I had never heard of. Typically, light rain is falling when the values reach approximately 20 dBZ. As you can see from my map, I was nowhere near that. I suppose that the high clouds or other particulates in the air could have accounted for the return that I saw on the map, but it certainly wasn’t raid. Had I known about the two scales, and the 20 dBZ threshold, I wouldn’t have been confused!
There’s quite a bit to learn about weather radars and how they are used to predict the weather. I would highly encourage you to visit the NWS Radar Image WSR-88D Radar FAQs to learn more. By understanding the concepts, scales and technologies used to predict the weather, you can get a better understanding of the weather potential for your area. And, it never hurts to learn some geeky science!
