These effects are collectively known as tropospheric propagation, or tropo for short. Tropospheric propagation can affect wireless communications, sometimes enhancing the usable range, but also compounding interference problems. The most well-known form of tropo is called bending.
Air reduces radio-wave propagation speed compared with the speed in a vacuum.
The greater the air density, the more the air slows the waves, and thus the greater is the index of refraction. The density and index of refraction are highest near the surface, and steadily decrease with altitude. A wave beamed horizontally can follow the curvature of the earth for hundreds of miles. The lower atmosphere scatters electromagnetic radiation over a vast range, including radio wavelengths. This effect is known as tropospheric scatter, or troposcatter.