Effects of Mountain Topography on Surface Winds

Mountains represent the maximum degree of surface roughness and thus provide the greatest friction to the general surface airflow. Mountain chains are also effective as solid barriers against airflow--particularly dry, cold air of polar origin and relatively cool Pacific marine air. While warm, light air may be forced aloft and flow over the ranges, cool, heavy air is often either dammed or deflected by major mountain systems.

Over short distances and rough topography, gradient balance may not be established and winds of considerable speed may blow almost directly across isobars from higher to lower pressure. Winds of this nature are common in both coastal and inland mountain regions. This type of flow is particularly noticeable in the strong pressure-gradient region of a Santa Ana pattern.

Mountains and their associated valleys provide important channels that establish local wind direction. Airflow is guided by the topography into the principal drainage channels. Less-prominent features of the landscape have similar, though smaller scale, local mechanical effects on wind speed, direction, and turbulence. In short, winds blowing over the surface are influenced by every irregularity.

In addition to these mechanical effects, strong daytime convective activity in mountain areas often alters or replaces the general wind at the surface. General winds are most pronounced at the surface in the absence of strong heating.

Canyon Eddies

General winds blowing across mountain ridges are lifted along the surface to the gaps and crests. If the air is stable, it will increase in speed as it crosses the ridge. Ridgetop winds thus tend to be somewhat stronger than winds in the free air at the same level. How the air behaves on crossing a ridge is influenced by ridge shape and wind speed and direction. Round-topped ridges tend to disturb surface airflow the least. In light to moderate winds there is often little evidence of any marked turbulence. Sharp ridges, on the other hand, nearly always produce significant turbulence and numerous eddies on the lee side. Some of this is evident at the surface as gusts and eddies for short distances below the ridgetop, though much of it continues downwind aloft. Wind blowing perpendicular to the ridge line develops the least complex wind structure downwind, and most of the eddies formed are of the roll or horizontal type. If the angle of wind approach deviates from the perpendicular by some critical amount, perhaps 30° or less, vertical eddies are likely to be found in the lee draws below the ridgetop, in addition to eddies in other planes.

Eddy currents are often associated with bluffs and similarly shaped canyon rims. When a bluff faces downwind, air on the lee side is protected from the direct force of the wind flowing over the rim. If the wind is persistent, however, it may start to rotate the air below and form a large, stationary roll eddy. This often results in a moderate to strong upslope wind opposite in direction to that flowing over the rim. Eddies of this nature are common in the lee of ridges that break off abruptly, and beneath the rims of plateaus and canyon walls.

Ridgetop saddles and mountain passes form important channels for local pressure-gradient winds. Flow converges here as it does across ridgetops, with an accompanying increase in wind speed. After passing through mountain saddles, the wind often exhibits two types of eddy motion on the lee side. One takes the form of horizontal eddies rolling or tumbling down the lee slope or canyon, although the main eddy may be stationary. The other is usually a stationary vertical eddy in one of the sheltered areas on either side of the saddle. Some of these vertical eddies may also move on downwind.

General winds that are channeled in mountain canyons are usually turbulent. The moving air in canyons is in contact with a maximum area of land surfaces. Alternating tributaries and lateral ridges produce maximum roughness. Whether the canyon bottom is straight or crooked also has an important influence on the turbulence to be expected. Sharp bends in mountain-stream courses are favorite "breeding grounds" for eddies, particularly where the canyon widens to admit a side tributary. Such eddies are most pronounced near the canyon floor and dissipate well below the ridgetop.

See: Mountain Waves and Foehn Winds