Apropos of nothing,
Anyway, here’s a question about cold. Currently, it’s 36 F degrees in Valley Village, CA. Yet I expect there to be frost on the roof of my house at 7 AM, even though the temperature won’t quite reach the freezing point of 32 F. When I was a kid in the San Fernando Valley, it seemed to get down to 28 F pretty regularly, but lately freezing temperatures are rare. As far as I can tell, the last time it got as low as 31 degrees F was December 28, 2015.
And yet in both December 2015 and December 2016, my Brazilian Bougainvillea bush was blasted by frost (recovering nicely by spring, fortunately).
My impression is that locally extremely low temperatures roll down off the roof onto the bushes. But how does that work? How does it get colder in some spots in my yard than the overall temperature?
A physics professor once explained this to me as a consequence of different types of heat transfer. Your roof and yard are not only attempting to match temperatures with the ambient air, although that constitutes the majority of the heat transfers going on. If the night sky is cloudless, they are also radiating heat into deep space. If there’s cloud cover or a canopy over the terrestrial surface, then that covering will radiate some heat back as a black body and the transfers more or less cancel. However, there is no such parity with deep space–heat radiated into the void is gone for good with no recompense. This can make a couple of degrees’ difference.
Looks like some people in the thread noted this as well. They also pointed out the much more obvious fact that cold air will pool in the lowest spot.