Editing Skywave (section)

== Other considerations ==

[[Very high frequency|VHF]] signals with frequencies above about 30&nbsp;MHz usually penetrate the ionosphere and are not returned to the Earth's surface. [[E-skip]] is a notable exception, where VHF signals including FM broadcast and VHF TV signals are frequently reflected to the Earth during late spring and early summer. E-skip rarely affects [[UHF]] frequencies, except for very rare occurrences below 500&nbsp;MHz.

[[frequency|Frequencies]] below approximately 10&nbsp;MHz (wavelengths longer than 30 meters), including broadcasts in the [[mediumwave]] and [[shortwave]] bands (and to some extent [[longwave]]), propagate most efficiently by skywave at night. Frequencies above 10&nbsp;MHz (wavelengths shorter than 30 meters) typically propagate most efficiently during the day. Frequencies lower than 3&nbsp;kHz have a wavelength longer than the distance between the Earth and the ionosphere. The [[maximum usable frequency]] for skywave propagation is strongly influenced by [[sunspot]] number.

Skywave propagation is usually degraded &ndash; sometimes seriously &ndash; during [[geomagnetic storm]]s. Skywave propagation on the sunlit side of the Earth can be entirely disrupted during [[Sudden Ionospheric Disturbance|sudden ionospheric disturbances]].

Because the lower-altitude layers (the [[E-layer]] in particular) of the [[ionosphere]] largely disappear at night, the refractive layer of the ionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.