Air next to the mountain becomes warmer than that at the same level away from the mountain and over the plains. This produces a relative low pressure in the air next to the mountain. The low pressure induces convergence of flow toward the mountain. Air begins to flow up the slope. Provided there is sufficient heating and low stability, the airflow will eventually rise over the peak and break away from the slope.
Updrafts then form over the mountain.
An illustration of this mechanism is depicted in Fig. The addition of water vapor to the atmosphere from evaporation and evapotranspiration at the surface of the mountain slope creates enhanced buoyancy effects. In addition, the moisture is a critical component for the formation of clouds. The picture is somewhat different for unstable airmasses. The effects of mountains noted for stable orographic clouds apply to unstable clouds as well.