What is MUF or Maximum Usable Frequency?

Maximum usable frequency (MUF) is dependent on the angle of incidence θ. MUF = critical frequency/cosθ. Critical frequency is the maximum frequency at which total internal reflection occurs from the ionosphere. Higher frequencies travel further outwards without getting reflected back to the earth. There is also an optimum working frequency or OWF which is 0.85MUF. Maximum usable frequency and optimum working frequency are with reference to communication between two given points at given a time.
The thread on qrz.com menioned that 20 m was active often until midnight, and even 17 and 15 m could last well into the dark hours, till a few months back. But he noted that MUF is often 8-10 MHz by 9 pm now. Other participants in the discussion mentioned that there is regional variation in the MUF. A dynamic MUF map is available online prop.kc2g.com/. The map updates every 15 minutes. Interestingly I found MUF about 28 MHz in my region when I checked at 9.30 pm local time! Maximum was noted at 38 MHz where it would be day time. There were also regions with MUF well below 10 MHz at other places where it was night. As you can see, there are also zones of lower MUF within areas of higher MUF.
Web: johnsonfrancis.org/techworld/...