Tropospheric ducting (also known as tropospheric refraction) is a type of radio propagation that tends to happen during periods of stable, anti-cyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (though some people have received "tropo", as it is usually abbreviated, from beyond 1000 miles), while with tropospheric bending, stable signals with good signal strength from 500+ miles away are not uncommon when the refractive index of the atmosphere is fairly high.
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