Composite reflectivity is a radar display that shows, at each horizontal location, the highest reflectivity found anywhere in the vertical column above that point, collapsing the full volume scan into a single map of maximum echo.
How the product is built
Each volume scan produces reflectivity data at a series of elevation angles. At every horizontal grid cell the algorithm walks up the column, checks the reflectivity value at every altitude the radar sampled, and keeps the largest one. That single maximum is written to the composite image. Areas the beam never sampled, either because they are in the cone of silence directly above the radar or below the lowest tilt at long range, are filled from the nearest available slice.
The result is a two-dimensional map that answers a specific question: what is the strongest echo anywhere overhead. That single question is the reason the product exists and also the source of every way it can mislead.
How forecasters use it
Composite reflectivity is the first-glance product for hail-supporting cores. Deep, strong updrafts loft large hydrometeors well above the freezing level, and those elevated cores produce reflectivity values above 55 or 60 dBZ that a low-tilt image cannot see at all. When a composite lights up a small area at 65 dBZ while the base reflectivity at that same spot is only 40 dBZ, the difference is telling the reader that a strong core sits well above the surface and is being actively held up by an updraft.
It is also useful for tracking coverage across a large domain. On a busy convective day, a composite mosaic across many radars gives a fast picture of where the strongest cells are without requiring the operator to click through elevations on every one. That situational overview is how national and regional composites are typically used at the forecast desk.
Where composite reflectivity misleads
The product intentionally destroys the vertical structure it summarizes. A location that shows 60 dBZ on composite may be reading a core near the surface, a core high in the mid-levels, or anything between. Storm-mode diagnosis and structure work cannot be done on composite, because the shape of any single layer is lost in the maximum. Hooks, notches, and inflow bands are read on base reflectivity, not on composite.
Composite values can also be inflated by beam sampling geometry. When multiple radars overlap, the algorithm typically keeps the largest return from any contributing radar, which can shift the apparent core location or add echoes that only one radar was seeing. Reading composite honestly means treating it as a screen for where to look closely, then stepping back to base reflectivity and to velocity data to answer any question about what is happening at a specific height.