Definition
Thunderstorms that form when moist, unstable air is forced upward by sloping terrain such as a mountain range. As the air is pushed up the windward side of the terrain, it cools, condenses, and -- if instability is sufficient -- builds into a cumulonimbus cloud capable of producing lightning, heavy rain, hail, and strong gusty winds.
Plain English
Thunderstorms caused by air being shoved up a mountain slope. The rising air cools, forms tall storm clouds, and if the air is unstable enough, a thunderstorm develops over or near the high ground.
Context Anchor
Seen in aviation weather study, preflight weather briefings, and mountain flying, especially when clouds are building along ridges or slopes.
Derivation
From the Greek 'oros' meaning 'mountain' and '-graphic' meaning 'related to.' So 'orographic' literally means 'caused by mountains' -- a useful clue that any 'orographic' weather phenomenon is triggered by terrain forcing air upward.
Why Pilots Care
These storms can form quickly over terrain even when skies are clear nearby, creating sudden turbulence, icing, and downdrafts that are especially hazardous in narrow valleys or passes.
Grounding Statement
Picture moist air blowing toward a mountain, being forced up the slope, cooling as it rises, and building into a storm cloud.
Intuition Check
Do not assume the mountain alone creates the thunderstorm. The key is moist, unstable air being lifted by the terrain.
Example Sentence 1
The forecast warned of orographic thunderstorms along the western slope of the Rockies by early afternoon, so the pilot rescheduled the crossing for the following morning.
Example Sentence 2
Orographic thunderstorms often intensify in the afternoon as daytime heating adds lift to the upslope flow.