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We know that the primary source of interference at HF is atmospheric noise. Thus, all else being equal, the strength of heard noise is a fair indicator of RX system performance (such as in adjusting an antenna tuner for maximum noise while receiving, in lieu of transmitting and measuring SWR), since the noise is received from all over and is not local interference which it would be useful to remove.

Suppose that we leave the frequency, antenna tuner, antenna orientation, etc. unchanged, but observe a change in received noise power. Does this reliably indicate a corresponding change in propagation conditions, or does the absolute level of atmospheric noise change independent of propagation?

In other words, if (I'm not making any contacts and) my radio is quieter than it was an hour ago, is it time to change bands?

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First of all, noisier or quieter, if you haven't made any contacts, it's probably time to change bands, regardless of anything else.

As to the meat of your question, what does a quieter actually mean? Basically, it means one or more of the sources of RF noise has quieted down. What are these sources of noise? Well, here's a few:

  1. Aurora Borealis (Northern Lights)
  2. Thunderstorms (Mostly noticed in spikes on the air). Note, these could be far away.
  3. Local Electronic Interference- This will peak when people are awake, and die down during the quiet times of the day. It will also peak if you are receiving interference from a distance. The distance is relatively rare, mostly local RF noise is relatively close. Going out to a farm outside of the city will reduce your noise to almost nothing.
  4. Solar activity- The sun setting or a sun spot quieting down could cause this. If the Sun just set, it will almost certainly get quieter.

I'm sure I could come up with a list of other things, but the bottom line is, the band quieting down could very well be a good thing for making more contacts.

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    $\begingroup$ Add another noise source -- intentional transmitters. If propagation is good, you'll hear more people, and sometimes so many more you can't distinguish them. This varies both on propagation conditions and what contest is currently in progress. $\endgroup$
    – user10489
    Jun 19, 2020 at 11:50
  • $\begingroup$ Also, solar radiation increases propagation. But it also adds noise. So when a CME hits us, the noise floor goes WAY up, but so does propagation, and when the noise dies down, there will be excellent conditions for a day or so. $\endgroup$
    – user10489
    Jun 19, 2020 at 11:53

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