dBZ Max

The dBZ Max on the storm attribute table is the highest reflectivity found within a particular thunderstorm cell. Depending on the reflectivity, a radar operator can determine the rate at which rain is falling as well as the potential for severe hail.

Rain starts to fall when the reflectivity (dBZ) is around 20. Of course, a set of atmospheric conditions can alter this. A very moist and saturated environment could allow rain or drizzle at a lesser reflectivity level. However, a dry environment could allow rain to reach the surface only at a higher reflectivity, as some of the precipitation could evaporate before it hits the ground.

Snow falls at much lower reflectivity levels. The radar will likely not tag even a band of heavy snow, unless it is associated with convection, creating thundersnow, for the storm attribute table. In general, snow reflectivities are less than 30 dBZ.

A reflectivity (dBZ) of around 55 may suggest large hail is falling, but not always. For a better indication as to if hail is falling with a storm, compare the freezing level in the atmosphere to the size of the updraft tower. The lower the freezing level intersects with the updraft, the better chance there is of hail, given the updraft tower has been sustained. Referring to the vertically integrated liquid, or VIL, may also be a good idea.

On the radar image, one indication of rather large hail with a cell is a hail spike. A hail spike is shown extending radially outward from a radar site (pointing the opposite direction) when the radar beam crosses through the core of a thunderstorm.

A radar cannot detect the maximum reflectivity (dBZ) well, or possibly at all, if a thunderstorm is close to the radar site. Beams are unable to extend into the updraft at such a degree from the radar's transmitter.

The vertical height of the maximum reflectivity (dBZ) can also be determined.


Jordan Gerth, May 2007