The scale of dBZ values can be seen along the bottom of the image.
dBZ stands for decibels relative to Z. It is a meteorological measure of equivalent reflectivity (Z) of a radar signal reflected off a remote object. The reference level for Z is 1 mm6m−3, which is equal to 1 μm3. It is related to the number of drops per unit volume and the sixth power of drop diameter.
Reflectivity of a cloud is dependent on the number and size of reflectors (hydrometeors), which includes rain, snow, graupel, and hail. A large number of small hydrometeors will reflect the same as one large hydrometeor. The signal returned to the radar will be equivalent in both situations, so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image.
A meteorologist can determine the difference between one large hydrometeor and a group of small hydrometeors as well as the type of hydrometeor using the polarization and phase shifting of the Doppler Radar. The reflectivity image is just one type of image produced by the radar and a meteorologist cannot tell the difference between nickle sized hail and heavy rain. In combination with other images gathered by the radar during the same scan (dual polarization products), they can distinguish between hail, rain, snow, biologicals (birds, bugs), and other atmospheric phenomena.
One dBZ-scale of rain:
0-8 Barely anything
dBZ values can be converted to rainfall rates in millimetres per hour using this formula: