Radar attenuation is the weakening of a radar signal as some of its energy is absorbed or scattered on the way out to a target and back, so returned echoes are dimmer than a clean, hazard-free path would produce. It is a fractional reduction in power per unit path length, usually described in decibels per kilometer.
How energy is lost
Two physical processes drive attenuation. Absorption removes energy as it interacts with water molecules and other atmospheric constituents. Scattering redirects energy in other directions, so some of the outgoing pulse never reaches the target of interest, and some of the returning echo never makes it home.
The amount of loss grows with the density and size of the hydrometeors in the beam path. A distant storm hidden behind a nearer heavy core is being sampled through a lot of extra water, and every meter of that path takes another small bite out of the signal.
Why wavelength changes the story
Attenuation is strongly wavelength-dependent. Shorter-wavelength radars, like C-band systems commonly used on some television and research platforms, attenuate more heavily in rain than longer-wavelength radars. The S-band WSR-88D near 10 centimeters attenuates much less, which is one of the reasons it was chosen for the national operational network.
Even S-band can be attenuated by intense rain and by melting hail, and dual-polarization variables such as specific differential phase are used to estimate and correct for that loss in modern processing.
How forecasters compensate
The classic tell is a storm that looks weaker on the far side of a heavy rain core than physics would suggest, sometimes ending in a 'V' notch of quiet behind the core. Forecasters cross-check with a neighboring radar that sees the same downstream storm through a cleaner path, and lean on satellite and lightning trends to keep intensity honest.
Dual-pol products help too. When correlation coefficient stays high through a suspect volume, the softer reflectivity is more likely to reflect a real environmental change; when it dips and specific differential phase surges upstream, attenuation is a good candidate for the dimmer picture downstream.