For a long time, I didn’t understand the difference between composite and base reflectivity when using Aviationweather.gov. You can see the difference when you check our radar picture. However, this excellent article in AOPA Pilot about Datalink helped clear up the confusion (subscription required). The two images below show radar returns and the difference between composite and base.
“Composite reflectivity imagery shows an amalgam of all the tilt-angle scans. The goal is to depict the maximum echo intensities from any antenna sweep at any elevation angle. Think of a composite reflectivity image as a stacking of individual scan overlays, with each one showing the highest levels of radar 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.”
You can find the entire article here, as well as recommendations about what reflectivity is best for your flight planning.