Skip is a term associated with signals from the ionosphere. What causes skip?

"Skip" is the colloquial term for sky-wave propagation, which allows radio signals to travel well beyond the horizon. This phenomenon is caused by the refraction (bending) of radio waves as they travel through the ionized layers of the Earth's upper atmosphere, known as the ionosphere. Although we often say signals are "reflected" or "bounced" off the ionosphere, the actual physical process is refraction. As the wave enters the varying density of electrons in the ionosphere, its path is gradually bent back down to Earth. Options like local cloud cover or antenna gain are completely unrelated to ionospheric skip. Weather happens in the troposphere (much lower down), and while a good antenna helps you transmit a stronger signal, it is the ionosphere itself that actually bends the wave.