First, mountains are obstacles to atmospheric flow. If the necessary atmospheric conditions exist, particularly the stability and moisture content of the atmosphere, clouds and precipitation may be produced due to the forced-orographic uplift of the air. To a large extent, the stability of the atmosphere determines how the obstacle will affect the flow. In particular, the stability determines the maximum orographic lifting realized by the air. If the atmosphere is more stable, there will tend to be less vertical displacement and more flow around the mountains.
Mountains also cause blocking of stable flow.
Flow retardation can result in horizontal-mass convergence upstream of the mountain, and may produce upward air motion and cloud formation before the mountain slope is reached. Similar to mountain blocking is flow deflection by a mountain. Airflow deflection can also produce regions of horizontal mass convergence, but in a different location. Stable airflow may be forced to split and flow around the mountain or obstacle. The airflow then converges on the lee side of the obstacle and may create a region of enhanced instability.