Here's some references that are interesting...
https://www.isode.com/whitepapers/skywave.html ; talks about variations
over 10 seconds to a few minutes.
http://tracebase.nmsu.edu/hf/reports/walnut.pdf describes a model of HF
links.
And gives some statistics. D (decile value of variability) = 1.28*SigmaN
(SigmaN = SD of noise power)
3 MHz D is about 8 dB during stable hours and ranges from 10-17 during
transition
10MHz D is 5 dB during stable hours, 7-11 transition.
Shorter term variations (Rayleigh fading) - those are modeled by what's
called the Watterson model.
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