In the atompshere, there are alot of assumptions that go in the equations that tell us how things are governed and/or behaving. One of these things is referred to as the ‘standard atmosphere’ such that we assume that a radar beam gradually increases with height as its distance from the radar increases. This is because the earth is curved, and radars base elevation scan (typically) are about 0.5 degrees.
However, this tends to neglect the temperature and humidity of the atmosphere. Therefore, when there are strong inversions (temperature increases with height), the radar beam will actually duct towards the ground (due to physical wave laws) Therefore, you are seeing returns from the radar beam that are actually hitting the ground!