Definition
A daytime local wind that flows up a valley slope, caused by the sun heating the valley walls and floor. The warmed air becomes less dense and rises along the slopes, drawing cooler air up from the lower end of the valley to replace it.
Plain English
During the day, the sun heats the sides of a valley. The warm air rises up the slopes, and cooler air from below flows up the valley to take its place. That upward flow of air is the valley breeze.
Context Anchor
Seen in mountain flying and local weather discussions, especially when planning wind conditions near valleys, slopes, passes, and mountain airports.
Why Pilots Care
It can create updrafts along slopes and affect takeoff performance or turbulence at mountain airports.
Grounding Statement
On a sunny afternoon in the mountains, the slopes warm first, air rises along them, and the valley air starts moving uphill.
Intuition Check
Do not read valley breeze as just any light wind in a valley. In aviation weather, it means a daytime uphill or up-valley wind caused by heating.
Example Sentence 1
By mid-morning, a steady valley breeze had developed, giving us a smooth climb along the sunlit ridge.
Example Sentence 2
As the sun climbed higher, the valley breeze strengthened and produced gentle updrafts along the ridgeline.