I used the get about 100-150 aircraft during the daytime hours (ADS-B). It's gotten cooler out and it runs about 50, is this normal? I know there's less small private aircraft during temperature extremes, but that's not equal to 30%.
Can you test your setup temporarily on another service...like tuning it to NOAA or something? It's hard to tell where you are if there is actually less air traffic, or if the cold has things contracting in your feedline and is affecting the signal path.
Not sure why you're seeing fewer AC. I've been feeding Flightradar25 for six years now. Gets down to 40 below here the odd time in the winter. I've never noticed any change in reported AC.. We've been around 25 - 35 F for the last few weeks. Still getting lots of AC up to 200 nautical miles.
Easiest thing to do is to recall how old the coax is - it may be time to replace it. If you have to think hard about when it was last replaced - it is time for new coax. Assuming the coax is newer, check all the connections.
Make certain to seal all connections with weather resistant coax tape.
I have been feeding 1090 MHz ADS-B for around 10 years now and I have not noticed a difference in the winter. Well, actually a little bit more range with the lack of foliage, but not less. Something may be loose, or causing interference. I use an amplifier, but it also has a filter which I'm sure keeps the RFI I cause myself while TX out of it.
@AM909 The OP noted ADS-B, so they are monitoring signals directly from aircraft on 1090 MHz and or 978 MHz. They are not monitoring voice conversations, but rather data bursts that a computer program handles.