I hope I can help a little bit!
WX radar beams are not flat, they are however cone shaped.
Also, the smaller the radar antenna, the wider this beam is.
As an example, a flat plate 29" antenna on a B737NG will produce a 3.4º beam.
This may seem very little, however draw a couple of right angled triangles and you will see that there even if your antenna is pointing upwards, you may get some ground clutter.
WX radar beams are rather like that of a torch. Try it out, point a torch a a wall in a dark room. The closer you get to the centre of the beam, the more intense the light is, right?
Back to the 737 radar...... In just 1º of beam, you will fling out 60% of the energy. This means that only 39.9% of the remaining energy is spread out in the remaining 2.4º of the beam. (0.1% is energy that is lost in the process).
However, what is more reflective than a drop of water? Exactly, solid stuff like buildings, trees etc..... So, even though very little energy is giving such building a sun tan, most of that energy bounces off and returns.
So there you have a couple of simple and true reasons which I hope have helped you to figure out what is going on with you WX radar.
Fly safe. Avoid those CBs!
Alberto