Definition
Winds that flow down the side of a mountain or sloping terrain because the air in contact with the higher ground has cooled, become denser, and is pulled downhill by gravity. They typically form on clear, calm nights when the slope surface radiates heat away rapidly, chilling the air just above it.
Plain English
Air sitting on a mountainside gets cold at night, becomes heavier than the air around it, and slides down the slope into the valley below.
Context Anchor
Seen in mountain and valley weather discussions, especially for night, early morning, takeoff, landing, and low-level flight near sloping terrain.
Derivation
Downslope simply means down the slope. The key word is cold: the wind exists because the air has been chilled and gravity does the rest. Naming it cold downslope distinguishes it from warm downslope winds (like a Chinook or Foehn), which are driven by air being forced over a ridge and warming as it descends.
Why Pilots Care
These winds can produce abrupt drops in temperature and density altitude changes that affect engine performance, lift, and the risk of carburetor icing.
Analogy
Think of cold air like water slowly draining downhill. It collects in lower areas and can keep moving along valleys after it leaves the slope.
Grounding Statement
Picture a clear, still night on a mountain: the slope gives off its heat to the sky, the thin layer of air touching the slope cools, gets heavy, and slides down into the valley like cold water running off a tilted tray.
Intuition Check
Do not assume this means any wind that happens to blow downhill. Cold downslope winds are caused by cooled, heavier air draining down from higher terrain.
Example Sentence 1
On the preflight weather check, the briefer mentioned cold downslope winds were likely at the mountain strip after sunset, so we planned to land before dusk.
Example Sentence 2
Cold downslope winds increased air density and gave the aircraft better climb performance than forecast.