Precipitation is described as orographic when it is generated or enhanced by air that rises as it interacts with terrain. This interaction can act to increase precipitation amounts (for example, locally increased precipitation in the mountains of south Wales during moist southwesterly airflow). Alternatively, this interaction may generate precipitation in a location where none would otherwise occur.
As a parcel of air flows across hills or mountains, it will typically rise, expanding and cooling as it does so. If the parcel cools to its dewpoint temperature, water vapour in the parcel will condense, forming a cloud. As the parcel continues to rise, it can generate a layer of stratiform cloud blanketing the terrain. If this cloud contains sufficient moisture, precipitation may occur due to this orographically induced lifting.
Orographically induced stratus cloud can also act to increase precipitation amounts via a mechanism known as the seeder-feeder process. If precipitation from a large-scale cloud falls through the stratus, it can collide with and collect moisture in the stratus, increasing precipitation at the ground.
Terrain may also trigger convection, as solar heating of mountain slopes can help to locally enhance ascent. This can result in showers and thunderstorms, as occurs during summer in the desert south-west of the US, and can lead to devastating flash-flooding.