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