Cold air settles into low spots on a calm morning
On clear and calm nights, we often talk about how deep and sheltered valleys will likely be colder than ridges. These common fall and winter conditions makes for a tricky forecast, with a wide range of low temperatures in the early morning hours.
Valleys get colder than surrounding ridges on clear and cold nights because an inversion sets up. This is where the air near to the ground get colder than air above it. This happens because on clear nights with light winds, the ground loses heat through radiational cooling. The heat radiates out as longwave radiation and escapes into outer space. This causes the air closest to the ground to cool rapidly.
On a calm night with light winds, the colder air drains into lower areas like valleys. This is because of density differences between the very cold air near the ground and relatively warmer air above it. Sometimes you can see colder air that has drained into valleys via fog. The colder air in the valley cools and reaches the dewpoint. This is when air becomes completely saturated and condensation takes place, creating the fog you see. You can also sometimes see frost in lower valleys while hillsides remain above freezing.
Sometimes a southerly wind flow above the surface can also develop. The warm air rides up and over the colder air in the valleys at night. Ridgetops often warm much faster than valleys with this setup. When this takes place, I’ve seen temperatures below freezing in valleys while high elevations are in the 50s.
So get ready, it is the season for clear and calm nights with wide temperature ranges.