Why is cosmic-ray radiation more intense at high elevations than at sea level?

Cosmic-ray radiation is more intense at high elevations because the Earth's atmosphere acts as a shield against incoming primary cosmic rays.1

When primary cosmic ray particles arrive at the outer edge of the atmosphere, they collide with atmospheric nuclei. These collisions produce "secondaries"—a cascade of fragments and short-lived particles, such as neutrons and muons. As these particles travel through the atmosphere toward the surface, they continue to collide with other nuclei, which reduces their intensity.1

At higher elevations, there is less atmosphere above to absorb or scatter these particles, meaning fewer collisions have occurred to attenuate the radiation. By the time the particles reach sea level, the intensity of the original cosmic radiation has been significantly diminished by the depth of the atmosphere it has traversed.1