Why can't radar's decibel scale be used to determine the intensity of snowfall? According to the FAA's Aviation Weather Services advisory circular,


  Values below 15 dBZ are typically associated with clouds. However, they may also be
  caused by atmospheric particulate matter such as dust, insects, pollen, or other phenomena.
  The scale cannot be used to determine the intensity of snowfall. However, snowfall rates
  generally increase with increasing reflectivity.


Why can't that scale be used to determine the intensity of snowfall?
Snow, unlike rain, has holes as well as different refractive properties. This allows the radar beam to penetrate and reflect the beam differently than an ordinarily oblate rain drop.