WXR Operation
The weather radar system will collect returns from precipitation to activate and acquire a paint the precipitation has to be moving and measurable by Doppler Effect. The higher the Doppler Effect the faster the precip is moving allowing the different colours to be displayed.
It will not pick up hail very well as it is a 'stealthy' circular shape allowing only a single point on the spherical hail to provide a return.
The radar range is affected by the time the radar beam travels out to target & returns so for the same measurable display the radar gain has to be increased as the returning signal is weaker. (STC Sensitivity Time Control)
IF the beam is entirely reflected by Extreme precipitation the backside display of the weather Can look clear (Black) the crew temptation would be to cut across the thin line of 'bad' weather to reach the calm beyond.
What is actually beyond is Hell.
Updated radars detect this as A PAC alert (Path attenuation Compensation) Along with The TURB Mode & PWS these are calculated figures within the radar.
They are best guess logic only but best guess based on years of experience and engineering & meteorological calculation.
The radar pic presented to the crew Has to be interpreted; the same display in Iowa has to be treated differently than in Hong Kong.
As a side note this is the second Airbus down with what pre investigation appears to be erroneous sensor data (ANZ A320 AOA - admittedly with apparent crew contribution)