Ascending air flow caused by mountains. Mechanisms that produce the lifting fall into two broad categories: 1) the upward deflection of horizontal larger-scale flow by the orography acting as an obstacle or barrier; or 2) the daytime heating of mountain surfaces to produce anabatic flow along the slopes and updrafts in the vicinity of the peaks. The first category includes both direct effects, such as forced lifting and vertically propagating waves, and indirect effects, such as upstream blocking and lee waves. Even though this term strictly refers only to lifting by mountains, it is sometimes extended to include effects of hills or long sloping topography. When sufficient moisture is present in the rising air, orographic fog or clouds may form.