Soaring pilots often learn about the basic concepts of valley wind formation and evolution early on in their flying careers. Those basics are: the wind in the valley flows up the valley during the day and a lighter reverse flow occurs at night. A rule of thumb when flying in the mountains is therefore that “rivers flow downstream, valley winds flow upstream.”
A valley wind is a diurnal thermally-driven circulation that is highly predictable in its onset, strengthening, dissipation, and reversal on synoptically benign days. This means that when there is an anticyclone, or high pressure over your region the local and plain-to-mountain circulations are driven almost exclusively by the sun’s daily passage across the sky and the radiative cooling at night.
If a similar airmass sits over your area from day to day, and if there are no significant high clouds, then the circulations that are a result of the sun’s heating of the terrain will turn on like clockwork.
Valley winds spill over
Typically, as the overnight inversion burns off, an up-valley wind begins to blow as a response to a pressure drop over the mountains as compared to the surrounding plains. The reason for this pressure drop is best thought of in terms of volume. The mountains take up...
