Base reflectivity is the reflectivity return from a single radar elevation angle, most often the lowest tilt in the volume scan, presented without vertical compositing so the operator sees exactly what the beam is sampling at that height and range.
How the product is built
A WSR-88D radar completes a volume scan by rotating the antenna through a sequence of elevation angles, from about 0.5 degrees near the horizon up through the mid-teens. Each elevation slice produces its own reflectivity field. The base reflectivity product plots one of those slices directly, without combining it with any of the others. In common operational use the lowest elevation is the default because it is the closest beam to the surface.
Because the display is a single tilt, every pixel corresponds to a specific altitude that increases with distance from the radar. Near the site the beam is only a few hundred feet up. At 100 nautical miles out the same 0.5 degree tilt sits well above 10,000 feet, so a single reflectivity image is really a curved slice through the atmosphere, not a flat map of the ground.
How forecasters read it
Base reflectivity is where storm structure is diagnosed. Hook echoes, forward-flank precipitation shields, bow apexes, rear-inflow notches, and the fine detail of an inflow band all live on a low-tilt reflectivity image. Because no compositing hides the shape of any single layer, a supercell hook that shows up on 0.5 degrees is a real feature of that layer, not an artifact stacked on top of unrelated echo above it.
Comparing successive tilts in the same volume answers a second question, which is how the storm is oriented in the vertical. A reflectivity core that leans sharply over the low-level inflow indicates a tilted, sheared updraft that can hold precipitation off the low-level inflow. A stacked, vertical core suggests a weakly sheared pulse storm. Both diagnoses come from stepping through base reflectivity at successive elevations.
Where base reflectivity misleads
The most common mistake is treating a base image as a surface picture. A storm 90 miles from the radar can be producing hail at the ground while the 0.5 degree beam samples cirrus outflow well above the core, or a distant strong echo can be entirely elevated in the mid-levels. The vertical growth of the beam with range also blurs fine structure that would be resolved at short range.
Base reflectivity also shows only one layer of the storm at a time. A high-reflectivity core aloft that a composite product would flag can be hidden on a low-tilt image. Reading base reflectivity honestly means knowing the beam height at the range in question and, when the answer matters, stepping through the higher tilts or checking a composite product before deciding what is at the ground.