Distance between arduinos

wvmarle:
I think the question should be: why bother?

Air pressure also changes the speed of sound. Higher pressure means faster sound.

I used to think that.

Speed of sound at sea level 0C is the same as at 5000M at 0C.
I picked that up learning amateur aerodynamics (made gliders, found out more than needed of course).

But don't believe me, go surf the net....
https://www.grc.nasa.gov/www/k-12/airplane/sound.html

I can tell you that humidity has an effect that's at least an order of magnitude less than temperature.

At below 10F I can clearly tell that sound is slower than above 32F right at ground level the day before.