What type of propagation is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis?

Explanation

Tropospheric ducting traps VHF and UHF signals in a layer of the lower atmosphere, so they follow the earth's curvature for a few hundred miles instead of escaping into space. Ranges near 300 miles occur regularly, especially over water and in stable weather.