Why valley bottoms are colder
On a clear, calm night, the coldest air in a landscape ends up at the bottom of it. The mechanism is simple and the consequences are not.
Surfaces radiate heat to a clear sky and cool. Air in contact with them cools too, becomes denser, and flows downhill. It collects in the lowest available basin and stays there, because there is nothing to mix it. The result is a temperature inversion: colder at the bottom, warmer partway up the slope. Differences of ten degrees over a hundred metres of elevation are unremarkable, and larger ones happen in enclosed basins.
The band of warmer air on the slope has a name in orchard country: the thermal belt. Fruit growers have planted with it in mind for a long time, because it is often the difference between a crop and a frost.
Two things break the pooling. Wind mixes the column. Cloud cover reduces the radiative loss that starts the process. So the coldest valley nights are the calm clear ones, which is also when the sky is most inviting to be out under.