I have had some conflicting experiences relating to the effect of night time (ionosphere, temperature, etc.) on propagation. Perhaps it is related to the frequency of what I was working with.
Several years ago I troubleshot a Wi-Fi network in the desert. Every night at a variable time (but early on in the night, say 10PM) the network would go down. I fixed this problem by using directional antennas. This was a ~2.45 GHz system. So it seems the received power, or at least SNR, decreased with night. One person I asked about it thought that it was due to increased noise (due to reflection of many other signals off the ionosphere at night).
Last night I looked at a broadcast FM station. The power was pretty high. I just checked it out again during the day and the receive power was at least 5 dB less. This was ~85 MHz.
So there are conflicting results. One signal propagates better at night, another worse.
Question: What things affect VHF/UHF signal propagation at night?