Troposcatter:

At frequencies above around 100 MHz, the environment has a dispersion effect on radio waves. The dispersion permits over-the-horizon communication at VHF, UHF, & microwave frequencies. This is termed as tropospheric scatter, or troposcatter. Clouds and dust in the air raise the scattering effect, though some troposcatter takes place regardless of the weather. The troposcatter occurs mostly at low altitudes where the air is the densest. Some effects take place at altitudes up to around 16 km (i.e., 10 mi). The troposcatter can provide reliable communication over distances of numerous hundred kilometers whenever the suitable equipment is used.

Figure below shows tropospheric bending and scatter. The transmitting station is at the lower left side. There is a temperature inversion in this illustration; it overstresses the bending. When the boundary among the cool air close to the surface and the warm air above is well defined sufficient, reflection can take place in addition to the bending. When the inversion covers a large geographic region, signals can bounce repeatedly among the inversion boundary and the surface; provide exceptional long-range communication, especially when the surface is salt water.

Figure: The troposphere can bend and disperse radio waves at several frequencies.