Tropospheric ducting occurs when a radio signal is reflected off of the troposphere and continues on a path that allows the signal to travel much farther than it normally would. This occurs when the temperature in the atmosphere experiences a shift called an inversion. When a temperature inversion occurs, radio waves that would normally continue into space beyond the Earth’s atmosphere are instead reflected and continue to follow the curvature of the planet. Radio waves have been able to travel in excess of 1,000 miles (about 1,600 km) because of tropospheric ducting.
The Earth’s troposphere is the lowest layer of the atmosphere. It extends from 4 to 40 miles (about 6 to 60 km) above the surface. During normal conditions, this layer allows radio waves to pass through into the upper atmosphere. Other layers, such as the ionosphere, prevent waves from being reflected higher up and returning to the ground.
During times of meteorological instability, the properties of the troposphere can change. When cold air that is low to the ground has a warmer air mass pass over top of it, it causes a condition called a temperature inversion. The cool air near the ground is moving slower than the warm air. This means radio waves that encounter a temperature inversion will be carried faster over the cold mass, bending the path of the wave downward and allowing it to curve with the surface.
Periods of relatively calm weather with clear skies are when tropospheric ducting is most often experienced, which is indicative of the high pressure fronts that can cause temperature inversions. The air masses have a high refractive index at this time, causing the radio waves to move more slowly and aiding in the bending of their trajectory. The actual landscape between the source of the signal and the horizon also can affect the distance it can travel, with flat land and water being the most effective.
There are other natural occurrences that can cause tropospheric ducting. Cool water coming off of a body of water under sun-warmed upper air masses can be a cause. In some areas of the Mediterranean, the effect can last for months at a time.
An entire branch of enthusiasts has developed around tropospheric ducting and, more broadly, tropospheric propagation. People who attempt to receive signals from long distances via tropospheric ducting are called DXers. The term comes from the radio code DX, which stands for distance. DXers are classified by groups depending on the type of signal they are attempting to detect. These signals can be radio, ultra-high frequency (UHF) or very-high frequency (VHF).