[ADTech]

Might be something as mundane as electrical interference from power lines or A/C equipment. You'd really need to hook up a spectrum analyzer and view the signals in order to do anything more than guess. My guess is that it's a combination of factors, neither of which specifically include what is most commonly referred to as tropospheric propagation although the underlying mechanism is closely related.

During the sun-heated hours, the atmosphere's noise level increases from the temperature increase. The increase in the noise floor will eventually swamp out the very weak signals that are just at or above the noise floor of the atmosphere when it's closer to quiescent levels.

The other possible factor is due to the same effect that causes visual mirages. The atmosphere just above the ground, when heated by the sun's energy, can form localized inversion layers that distort the signal path for the incoming signals, causing those signals to refracted or reflected elsewhere than your antenna's location.

Countermeasures for both are the same. A high gain antenna mounted as high as possible away from local noise sources along with a low-noise amp to preserve the system noise figure as seen from the antenna's terminals. That's about as good as it can possibly get.