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. The greatest communications range can be realized over flat land or over water.
Scattered waves are weak, so high-power transmitters and sensitive receivers are necessary.
A less common, but often dramatic, form of tropo is called ducting or duct effect. This occurs when there is a defined, horizontal boundary between air masses having different densities. Because radio waves are also reflected from the earth's surface, the result can be efficient propagation for hundreds or, in some cases, upwards of 1,000 miles, as the waves alternately bounce off the frontal boundary and the surface.