Why does radio reception change drastically at dawn and dusk?

[anorlunda]

i have noticed in several geographically diverse locations, drastic changes in FM radio and WiFi radio communications at the time of sunrise/sunset plus/minus a few minutes. But soon after the rise/set, they return to normal. Other frequencies, like cell phone reception seem unaffected.

I should not be surprised, but I am curious. Why the changes?

 

[Bystander]

FM is "line of sight," which varies with atmospheric conditions, which change radically at dawn and sunset due to temperature changes in some parts of the world.

 

[davenn]

FM radio

if the transmitters are some distance from you (not just across town) then you can see effects of morning and evening temperature inversions

WiFi radio communications

WiFi ? that's extremely short distance a few metres to a couple of 10's of metres ( give or take) ... couldn't imagine any effects at those times or any other time of day ... only thing that will affect WiFi signals will be obstacles between you and the router ... building walls

Other frequencies, like cell phone reception seem unaffected.

if within good reach of a cell tower, automatic gain control ACG within the system will keep a reasonably constant signal level
if you are on the fringe of the service area, all bets are off and anything will affect signal strengthDave

 

[anorlunda]

I should have mentioned that I always work with marginal signals.

My FM radio has no external antenna and is mounted only one foot above the ocean water level in my boat. Therefore, all signals are weak.

The WiFi spots that I have trouble with are 100-300 yards away on shore. Way beyond the specified range for WiFi. They work good enough to stream video days and nights, but not at dawn/dusk. At dawn, it drops out and I fire up my phone hotspot instead.

 

[davenn]

I should have mentioned that I always work with marginal signals.

My FM radio has no external antenna and is mounted only one foot above the ocean water level in my boat. Therefore, all signals are weak.

The WiFi spots that I have trouble with are 100-300 yards away on shore. Way beyond the specified range for WiFi. They work good enough to stream video days and nights, but not at dawn/dusk. At dawn, it drops out and I fire up my phone hotspot instead.

ahhh OK puts a new light on it ... it pretty much would confirm the temperature inversion layering that can occur at those times of the day

Sometimes it can be a good thing, us hams use ducting along coast lines caused by inversions and VHF to microwave signals can be greatly enhanced for may 100's to several 1000 kms at times

but in your case you are probably going at right angles to the inversion and in that case it's degrading your commsDave

 

[tech99]

Regarding the FM reception, many transmitters and receivers can employ both V and H polarisation. The vertical polarisation is not weak near the water - it does not drop with height over the lower few metres, due to the presence of a surface wave. I can observe this also with my marine VHF portable - signals are the same even when it is touching the sea.
Fading at dawn and dusk might be the result of an inversion layer or other ray bending. The sea normally has a layer of high humidity lying over it extending up to a few metres, and microwave links are always subject to propagation disturbance if the path is over the sea. In addition, the tide going up and down can alter signal strength due to the geometry of the reflected wave and I suspect that is what you are seeing with your WiFi.