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There are actually multiple phenomenon that are at play.
Refraction is the process of radio waves bending due to the atmosphere acting like a curved lens.
Tropospheric ducting is a phenomenon that mostly happens in evenings/nights when there is a thermal inversion layer (warm air near ground, cold air on top) that help radio waves follow the curvature of the Earth and thus travel further than normal "line-of-sight" distances.
Diffraction is the effect where radio waves "bend" around solid edges like a knife edge or mountain top. Even when there is a solid object in the way (like a mountain), some amount of RF energy is able to bend around those objects to fill in some of the space that would normally be in the "shadow" of that object.
All of these effects can vary over time (e.g., time of day, seasonal changes, etc.) or with changing conditions (e.g., humidity, rain, temperature, etc.).
The signal strength models used by TV Fool already account for these factors, but they only show the "average" situation. Your particular location, current weather conditions, time of day, and surrounding environment may cause you to experience higher or lower signal levels than predicted.
In general, all the "large scale" propagation effects are already factored into the numbers, so there's no need for you to make any adjustments yourself.
The models do not include details like individual buildings or trees, so you do have to take that into account yourself if you have something like that directly in the path of your antenna.
If you want to get an idea of well signals can get around obstructions (mountains and the curvature of the Earth), you can click on any of the rows in the channel list in your TV Fool report. That should bring up a new window showing a "cross section" view of the terrain between the transmitter and your location. The transmitter is always shown on the left and your location is on the right.
You can see how each mountain casts a "shadow" behind it. The signal gets weaker as you get "deeper" into any of the shadows because signals can only diffract (bend around objects) so far.
Given the readings on your report, it doesn't seem likely that you'll be able to get the Mount Wilson signals reliably. The transmitters are about 70 miles away, and you have some local mountains that are in the way. The signals reaching your location are very weak.
What kind of picture(s) are you getting when you tune to these channels?
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