Signals can come and go quickly or stay in all day.
Signals can be very strong or extremely weak. The signals can all be from the same geographical location, or they can be spread out all over the country. This happens even if the solar flux is low. It is sporadic-E skip, sometimes called E skip, and sometimes just short skip. The exact cause of these patches is not well known, but they are associated with wind shear and some think they are connected with high towering thunder storms.