http://www.airbus.com/store/mm_repos...V_WX-SEQ07.pdf
Of particular interest from this note is (emphasis mine):
"It is important to note that reflectivity of particles is not directly proportional to the hazard that may be encountered in a cell. Air can be very humid, when close to the sea for instance. In this case, thermal convection will produce clouds that are full of water. These clouds will have a high reflectivity, but will not necessarily be a high threat. On the other hand, there are equatorial overland regions where converging winds produce large scale uplifts of dry air.
The resulting weather cells have much less reflectivity than mid-latitude convective cells, making them much harder to detect. However turbulence in or above such clouds may have a higher intensity than indicated by the image on the weather radar display."