What causes the maximum usable frequency to vary?

The Maximum Usable Frequency (MUF) represents the highest frequency that can support communication between two points via ionospheric refraction. Because the ionosphere's ability to bend these signals depends on how heavily it is ionized, the MUF is directly tied to the amount of solar radiation received from the sun. As solar radiation increases during the day or during solar maximums, ionization levels thicken and the MUF rises. Earthly weather, wind speeds, and tropospheric temperatures do not dictate ionospheric electron density.