Radio Wave Propagation
The propagation of radio signals in the different radio bands is very much influenced by the condition of the sun, through the emissions of the sun, and also by the magnetic field of the Earth that captures them. Because of its importance for certain radio communications, a number of observatories continuously monitor the condition of the sun and the geomagnetic field. Here is a nice graph that tells you instantly how propagation fairs for today.
Maximum Usable Frequency
From Wikipedia: Maximum usable frequency (MUF) describes, in radio transmission, using reflection from the regular ionized layers of the ionosphere, the upper frequency limit that can be used for transmission between two points at a specified time, independent of transmitter power. This index is especially useful in regard to shortwave transmissions.
MUF is a median frequency applicable to 50% of the days of a month, as opposed to 90% cited for the lowest usable high frequency (LUF) and the Frequency of optimum transmission (FOT).
The graph shows today’s MUF reflections.