Images courtesy of NWS SRH Jetstream
In addition to beam spreading, the beam also does not travel in a straight line. The beam is bent due to differences in atmospheric density. These density differences, caused by variations in temperature, moisture, and pressure, occur in both the vertical and horizontal directions and affect the speed and direction of the radar beam.
The more dense the atmosphere the slower the beam travels. Conversely, the less dense the atmosphere the faster the beam travels. These changes in density can occur over very small distances so it is common for the beam, as it spreads, to be in areas of different densities at the same time. The beam will bend in the direction of the slower portion of the wave.
The atmospheric density naturally decreases with increasing elevation and is primarily due to the decrease in air molecules, and consequently air pressure, as elevation increases. This means the top portion of a beam in the atmosphere can move faster than the bottom portion. Under normal atmospheric conditions, when there is a gradual decrease of pressure, temperature, and humidity with height, a radar beam's curvature is slightly less than the earth's curvature.
However, atmospheric conditions are never normal. If the decrease in density with height is more than normal (the actual density is less than normal) then the beam bends less than normal and climbs excessively skyward. This phenomenon is known as subrefraction . Subrefraction causes the radar to overshoot objects that would normally be detected. Distant thunderstorms might not be detected with subrefraction as well as under reporting the intensity as the beam hits only the top portion of the thunderstorm cloud.
Conversely, if the decrease in density with height is less than normal (the actual density is greater than normal) then the beam bends more than normal and is curved more toward the earth's surface. This phenomenon is called superrefraction . Superrefraction causes the radar beam to be closer to the earth's surface than what would occur in a normal atmosphere. This can lead to overestimating the strength of a thunderstorm as the beam would be detecting more of the core of the storm versus the weaker upper levels.
If the atmospheric condition that causes superrefraction bends the beam equal to or more than the earth's curvature then a condition called ducting, or trapping, occurs. Ducting often leads to false echoes also known as anomalous propagation or simply AP.