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.
- a is for rain 300 in summer, 200 in winter and 1800 for snow
- a is for rain 1.5 in summer, 1.6 in winter and 2.2 for snow