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[rstark18]

I have my cam set to PIR detection and it's pointing down my driveway. I've noticed that during the day a car driving past will trigger a recording. At night though a car driving past will not trigger it. Why is that? It does trigger when I walk past the cam at night but why the cars during the day and not the night?

[JavaLawyer]

PIR measures radiation emitted by objects generating heat. This is conjecture, but I would think that since we're in the dead of winter, the exterior body of passing cars at night are very cold, i.e. close to ambient temperature, so little detectable radiation is emitted. The same car passing by during the day will absorb heat from the sun and have less heat loss to the warmer air, resulting in a detectable radiation signature. A person walking by in day or night will always generate heat and emit radiation.

     

[rstark18]

Makes sense. Thx

[JavaLawyer]

You'll have to wait till spring to test that theory, unless another user in a warmer climate can validate. 

[rstark18]

The temp here is 80 during the day and 55 at night.
 

[JavaLawyer]

The temp here is 80 during the day and 55 at night.
 

That pretty much invalidates my theory (perhaps)...