The radar horizon is the lowest slice of atmosphere a radar can actually sample at a given range, set by Earth curvature, atmospheric refraction, and terrain blockage. Below that height, the beam has already climbed out of the layer or been clipped by an obstacle, so nothing there is measured.
Why the beam leaves the surface
Even the lowest elevation angle in a WSR-88D scan strategy, typically 0.5 degrees, points slightly above the horizon. Combined with Earth's curvature, the beam rises steadily as it moves away from the antenna. At 100 nautical miles the center of the lowest tilt is already several thousand feet above ground level, and by 140 nautical miles it can be well over a mile up.
Standard atmospheric refraction bends the beam slightly back toward the surface, which extends the effective coverage a little. Nonstandard conditions can bend it more, or lift it further away from the ground, so the true radar horizon at any moment depends on the vertical temperature and moisture profile.
Terrain and the practical limit
Terrain places a hard floor under coverage. Mountains, ridges, and even large groves of trees can block the beam entirely along certain azimuths, creating persistent 'shadows' where no low-level data ever arrives. Site surveys published by the Radar Operations Center document these blockages for each installation.
This is why the NEXRAD network was designed with overlapping coverage. When one radar has a shadow behind a ridge, a neighboring site can often see into that same volume from another angle.
How forecasters read around it
Operational meteorologists mentally overlay the beam height on the storm layer they care about. A rotating updraft 90 nautical miles away is being sampled well above the surface, so the low-level circulation that matters for tornado risk may not appear until it approaches the radar or is picked up by a nearer one.
When a decision has to be made under a shadow or far from a radar, forecasters lean on surface observations, storm reports, satellite trends, and adjacent radars to fill the gap. Trusting a clear display beyond the practical range of a site is one of the classic ways to be blindsided.