"A primary radar pulse travels to an aircraft, bounces off it, and returns to the radar. This gives it two pieces of information:
- The direction (from the position of the antenna)
- The range (from the time the pulse took to return)
Secondary radar, on the other hand, effectively ""asks the aircraft a question"". The returning pulse is not the original one bouncing off, but an entirely new pulse transmitted by the aircraft containing, potentially, a lot more data."
As clouds and water droplets cannot respond to the interrogating signal, they do not show up.