I'm not an expert in the area, but I would say weather would affect reception just as it effects sound, as far as time of day I would say its always possible. Good question, its one I would like to have a great answer to.
I know when Conditions are "just right"I can hear quite a ways on the old trusty 780
I have lots of conventional freqs but now with dtrs...bummer A few years ago I could hear
155.3250 Elizabeth Fire
153.9950 Kiowa Fire
But I think they are both gone now,it is rare but at times I hear washington county Southwest Washington FPD from berthoud
I know all these have got to have something to due with weather
I'm not sure what the weather does to the scanners, but from a system users' standpoint, adverse weather wreaks havoc on several aspects of the system. If it's raining, the water clogs the microphone ports, if it's windy that's all you hear, if it's foggy the transmission are digital.
In summer, with very humid Nebraska nights, it is not uncommon to get NOAA WX broadcasts over 100+miles and FM Broadcast across the state and from Iowa, Kansas, Missouri, South Dakota. For example, my home stereo receiver last summer picked up KOGA, Ogallala, Ne 98.7, over 275 miles west of me, and hung in tough throughout the night. No fancy antenna, just a piece of wire hanging off the back of the antenna jack.
Iowa State Patrol, up on the VHF Hi neighborhood, from the east end of the state would make the trip into my BC296D very nicely, and South Dakota DTRS, rare, but I have logged them too.
Charles, I'm sitting at home in Broomfield with my GE MPA programmed to receive Aurora PD/FD. I usually get really clear reception where it's sitting now, but now that it's snowy the coverage is spotty/scratchy. So, I'm guessing weather does affect reception.
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I would believe denser air would improve the transfer of radio waves. However, I have no scientific evidence to support it. My reception of the 35th ave Greeley tower(I live off the Erie/dacono exit) seems better than usual since it has been cold.