Honda Civic Owner's Manual page 206

AM/FM Radio Reception
Radio signals, especially on the FM
band, are deflected by large objects
such as buildings and hills. Your
radio then receives both the direct
signal from the station's transmitter,
and the deflected signal. This causes
the sound to distort or flutter. This is
a main cause of poor radio reception
in city driving.
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Radio reception can be affected by
atmospheric conditions such as
thunderstorms, high humidity, and
even sunspots. You may be able to
receive a distant radio station one
day and not receive it the next day
because of a change in conditions.
Electrical interference from passing
vehicles and stationary sources can
cause temporary reception problems.
As required by the FCC:
Changes or modifications not expressly
approved by the party responsible for
compliance could void the user's
authority to operate the equipment.