An inversion layer (or duct) is most commonly observed over high pressure regions and may affect radio signals for several hours to several days.
In the USA, the Federal Communications Commission (FCC) propagation models used to space stations on the same frequency are not always accurate in prediction of signals and interference.
In the medium frequency portion of the radio spectrum where most AM broadcasting is allocated, signals propagate full-time via groundwave and, at nighttime, via skywave as well.
This means that during the nighttime hours, co-channel interference exists on many AM radio frequencies due to the medium waves reflecting off the ionosphere and being bounced back down to earth.
FCC deregulation allowed many new AM radio stations on the former clear and regional channel designations; this is the principal cause of overcrowding on the AM band at night.
The advent of 5G may significantly increase deleterious effects on satellites which would impair numerical weather prediction performance, resulting in substantially adverse economic and public safety impacts.
[2][3] Due to such concerns, US Secretary of Commerce Wilbur Ross and NASA Administrator Jim Bridenstine in February 2019 urged the FCC to cancel proposed spectrum bidding, which this was rejected.