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Reflectivity is the amount of transmitted power returned to the weather radar receiver. It is defined as the radar cross section per unit volume for small dielectric spheres. The reflectivity Z is proportionally to the number and the size of the rain drops in a volume unit. The unit of measurement is mm6·m-3. The reflectivity depends on a lot of factors and ranges over several orders of magnitudes, so it is usually expressed on a logarithmic scale called dBZ (decibels of Z, where Z represents the energy reflected back to the radar).

All dependencies are summarized to a factor and an exponent for meteorological use, as shown in this equation:

During a long-term monitoring a number of different values are ascertained for the factor a and the exponent b. A detailed description of these values are fixed for every radar (and every radar site) in an individual table.