I had not thought about the Bay. At the low angle of incidence w/respect to the surface, I'd expect primarily reflection, not absorption at the water surface. Do waves in the VHF range exhibit this normal behavior?
I thought you might pick up on some of this.
I gave a you clue, kind of like your science puzzles, when I mentioned the leaves on the trees and that the energy is scattered, refracted, reflected and absorbed on its path to your antenna.
Transpiration, I am sure you know what that is.
For others it is the giving off of water vapor by the leaves of plants.
Transpiration serves to evaporatively cool plants as the escaping water vapor carries away heat energy.
Light, temperature and relative humidity all will affect the rate of transpiration.
The more light, higher the temperature and lower the relative humidity the greater the rate of transpiration.
And Water lost from transpiration can and may accumulate in the boundary layer close to the leaf surface.
So what does plant biology have to do with FM radio reception?
The increased moisture in the air in the
"micro-climate" of the tree's canopy can and will have an affect.
Some of the signal passes through the plant material and is attenuated.
So when might one expect the rate of transpiration to be the greatest?
During the time of the highest temperatures, sunlight, and lower humidity.
This might also be the time of the greatest attenuation (reduction) in the field strength of the signal.
That is one mechanism.
With regards to the body of water, remember that the signal from the FM transmitter is a wavefront, is not a beam of energy with a single elevation.
And earth caused refraction, will in simple terms, cause the RF signal to
"crawl along the surface of the ground". To keep this simple for this example,it will be a given that there are no structures in the path of the RF signal.
As the signal crawls along in the area of the body of water it will encounter the shoreline. At that point it will encounter an impedance discontinuity. This discontinuity may change with the local micro-environment. There will be some reflection of the direct, incident RF signal, but remember that our RF signal is a wavefront and part of it is crawling along the surface, water and earth.
I mentioned the near radio horizon, but it is not necessarily a single distance, since the RF signal is a wavefront, not a beam.
The will be a weaker wavefront traveling to your antenna. Its field strength may vary due to the above conditions.
This wavefront will pass through various plants while it is still relatively close to the ground and at higher elevations it will pass through the leaves of the trees.
Which brings us back to transpiration and an increase in moisture in the micro-environment of the trees canopy. Again the brings us to variable RF signal path loses as the signal passes though this environment.
Remember that I mentioned that the RF signal is scattered, refracted, reflected and absorbed on it path to your antenna.
Plants, one of the universal disruptors of RF signals.
Of course if there are other structures in the path of the RF signal from the radio station, the field intensity prediction becomes that much more complicated.
This is a much simplified explanation.
Since I am not at your location, I offer this information in the most general of terms.
Below is a picture illustrating the vertical beam width of an antenna.
Note that this shows the signal applied to a flat surface, think about in terms of the curvature of the earth and the radio signal horizon and what I have mentioned above. This illustrates how the RF signal does not strike the earth at a single point.
The antenna for your radio station does not have any down tilt, the signal is radiated perpendicular to the antenna. That does not change anything relative to this discussion, other than making the -3dB point farther from the antenna and the point at which it first encounters the radio horizon farther from the radio station antenna.
