Couple of things needing explanation

Just a wild guess but maybe the rain increases the ground conductivity along the transmitter to receiver path.

Without doubting your observation I have noticed that while driving I can hear my AM signal pop in and out at a considerable distance from the transmitter rain or shine. It seems this is due to the power lines acting as a receive antenna and inducing the signal into the car antenna. This happens when I tune to a distant station. I wonder if that hot spot you found is there without rain.

Tuning of the antenna can be affected by moisture. Perhaps the tuning is slightly off such that the increased moisture brings the antenna system more in tune.

yep, that particular spot was heard when it wasn't raining, early evening, in fact, when the station usually starts getting stepped on by interference. There is a lamp strung on a tree just abeam that 20 ft. part of the lane.

Hmmm. 😉

First, it's nice to learn that I'm not the only one with my own station tuned on the car radio at all times, even when I'm way out of expected range 🙂

Second, I had that happen when I lived in Vancouver WA and it seemed to be caused by a creek. There were several points on my drive to work where a creek that flowed right by my transmitter location also flowed along my route.

In places where the creek was right next to the road, as well as on a bridge, the little AM station punched in. I think the most distant point was around 2.5 miles - way past anything I would have expected from the setup I had at the time (1999).

I often wonder what would happen if I had pounded a ground rod into the creek bed and set my XMTR & ANT right above the water 🙂

Yes, my wife humors me but eventually asks why we're listening to static.

But aint it great when across town suddenly your signal pops out of the static?

Last summer, my wife took me for a doctor's
appointment.

My AM station was running on 1690, and in
the parking lot at the doctor's, my station was
there. It was weak, but very clear and readable.
A map check showed that the reception was
exactly two miles from the house.

In my case, rain would detune my antenna system,
and the station would not do as well.

I also found that the station reception would come up
near shopping centers, or areas where there are lots
of buildings. My theory is that there were so many
buried grounds in those places, that they helped get
the signal to the receiver in conjunction with the above
ground wiring.

Anyway, it sure was fun.

Best Wishes,
Bruce, MICRO1690/1700

You know I have to chime in from time to time. 🙂

Before I put my SStran strictly on a piece of cable for in-house reception I was running a Talking House and had weak but readable signal up to 1/4 mile some places 1 mile in others.

At the intersection of US Hwy 192 and Old Whitley Rd. which is 1/3 of a mile from my home, the signal would jump way up as if i was sitting in my driveway listening. There you have your typical power lines feeding the traffic lights and numerous business's around that intersection.
Bob's Big Boy parking lot seems to have been the best place to listen because it was on the corner of this intersection.

Across the highway, nothing. If you were to drive East on Hwy 192 the signal would start out scratchy but somewhat listenable and would gradually get stronger as you traveled further away from my home.
It is probably more helpful to explain that hwy 192 is a main artery for traffic from I-75 so naturally there are a lot of retail stores, gas stations and hardware stores. Lowes is the biggest store in size compared to Walmart. At Lowes the signal was nearly perfect anything past that store, forget about it. Odd thing about it, i was using the wire antenna option and it was inside the house.
Also i think having heavy power lines leading from my place and traveling along the length of the hwy added to propagating my signal.

If it rained, the signal was lousy and usually was restricted to the trailer park. Radio proper-gates in mysterious ways, that's for sure.

This thread has opened for me the sense of a whole new mystery about low power AM, the idea that the signal might be out there doing unexpected things.

Yesterday I kept tuned in after the usual fade time while driving up the street and yes, the signal was there in places I did not expect, and although weak and noisy, I heard it for a long stretch of road.

What this suggests is that DXers have a chance, way off in the distance, of discovering our station!

Any chance you are driving over a substancial ground system. Maybe a buried steel pipe or power lines and your car is coupling to that ground system and achieving greater receive antenna effeciency? Just like the weak transmit antenna systems dictated by part 15 rules the car radio receiver is severly limited because of limited antenna and ground size. I'm sure directional antennas could be used on home receivers to make your station loud and clear to a listener 5 miles away. Unfortunately the receiver antenna would probably be 1 mile long itself!
The radio "system is all about signal to noise ratio and a car radio is at a severe disadvantage ( as is a table receiver) compared to a properly setup and tuned receiving system with a long outdoor antenna. I have owned two ford trucks with efi fuel systems and had to do extensive RFI troubleshooting and installation of toriods in the fuel pump leads to make AM at all usuable. Sometime when you start noticing your signal fading try pulling over and stopping the engine, put the ignition switch to accessory and notice how your station signal improves. Car radio installations are very marginal at best on medium wave. JMHO... Dave

Actually, I think that might account for it. There once was a floodlight on the wood sign down at the main road, but the lane has no overhead wires ... conclusion: Wires to the flood must be buried, from at least 70 ft. away, and are running across where the signal path would be. The taxi was probably closest to it when I turned the corner from the parking area lane, which would account for coupling right there, but then it got too far away.

As for car radios, in everyday common use, today many are far superior to anything else not using longwire antennas, have much better noise filters, and the car body provides some counterpoise, certainly much better than home stereo system receivers. IOW, I recommend tuning in on car radios in town. Virtually anything else in consumer hardware would require an outdoor antenna, or an indoor tunable one to be as good.

A longwire antenna for AM radio need not be more than 1/4 wave ... if you know the direction for station(s) you wish to hear, you could build a rhombic, but this not the audience I have or want.

OTOH, I have plans to build a nice-lookig decorative loop antenna system.