AOPA Pilot Magazine - August 2011 - 97

Composite reflectivity antenna scan angles
70 Height (thousands of feet) 60 50 40 30 20 10 0 0 20 40 60 80 Range (miles) 100 120
19.5 16.7 14 12 10 8.7 7.5 6.2 5.3

4.3 3.4 2.4 1.5 0.5

Advocates of composite reflectivity emphasize that it shows a better rendition of precipitation and convective systems as a whole. “You get the whole picture instead of a small slice,” said a representative of Baron Services’ WxWorx (WxWorx provides imagery to XM WX). Other proponents emphasize that composite reflectivity’s many sweeps deep into the flight levels give a better representation of a storm system’s true condition and potential. WSI chose base reflectivity for several reasons. “You can see the most important parts of radar reflectivity gradients

with base,” a WSI spokesman said. “Because of this you get a better snapshot of the weather at altitudes closer to the surface, where embedded conditions are most prevalent, and where more critical decisions must be made quickly.” Resolution of a base reflectivity image is more accurate because it’s the product of a single radar sweep, advocates say. With composite reflectivity, all those sweeps can “smear” the radar image during the time it’s compiled. This brings us back to the issue of time delay. Both base and composite reflectivity images may take five min-

Composite reflectivity antenna scans range from the lowest to the highest radar antenna scan angles. It takes longer for the radar to build an image made of so many different scans, and during the whole time the intensity of any convection can change. By the time a composite reflectivity image appears in your cockpit, thunderstorms may have moved or changed shape.

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tion, at longer ranges, the beams from the higher antenna tilt angles will only “see” precipitation returns occurring at the higher levels of the atmosphere. Building thunderstorm tops at levels below, say, 30,000 feet may go unrecorded, rendering them invisible on your map display. Base reflectivity On the other hand, base reflectivity shows the results of single sweeps made at the lower antenna tilt angles. Typically, this is the lowest tilt angle—a mere 0.5 degrees above the horizon. Base reflectivity is good for studying storm structure, identifying precipitation at lower levels of the atmosphere (say, below 20,000 feet), and seeing higher-resolution returns. Base reflectivity imagery can extend to ranges as far as 124 nm from the radar site. To see precipitation returns beyond that, you have to consult a neighboring Nexrad site’s results. That’s true for composite reflectivity images as well. That’s why the NWS—as well as private weather suppliers—create radar mosaics that show regional and national precipitation images. Which is best? Datalink providers XM WX Satellite Weather and WSI InSight offer pilots a choice. XM WX sends composite reflectivity images to the cockpit, while WSI’s InSight datalink service uses base reflectivity. The “best” service of choice has generated no small amount of controversy in some quarters.
AOPA PILOT • 97 • AUGUST 2011 Avemco_AOPAPilotMag_4.625x7.4375CalmAIRVENTURE.indd 1 6/15/11 5:51 PM

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AOPA Pilot Magazine - August 2011

Table of Contents for the Digital Edition of AOPA Pilot Magazine - August 2011