Motion Sensors and Fog

I figure yard lights are part of everybody's experience here, so hopefully somebody will have an idea!
Now that its 50 degrees and extremely humid foggy here in Milwaukee, I've begun having problems with my outdoor lighting.
At first I thought it was a wiring issue, perhaps shorting because of the wet... but the light works fine in "test" mode... just not regular motion mode. It also doesn't work when I simply try to turn it on with the "turn the switch off for 3 seconds, then turn it back on" permanent on mode. Then I thought maybe it was the light/dark sensor... but I noticed that the neighbor's automatic light hasn't been going on the for the couple days either.
So, I'm left with wondering if this is a normal thing, or if its something common to "cheap" lights, or if I should be worried my yard will burn down!
Thanks for any insight!

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