9th September 2005
Orographic Stratus
Source: Excerpt from The Book " Weather "
Distribution: Over hills and mountain ranges worldwide. Height: 0 to 1000 feet above ground level. Cause: Lifting of moist air mass by landform. Associated Weather: May produce fog, light drizzle or light snow showers. Hazard Warning: Can be an aviation hazard as it masks terrain. Orographic clouds are formed when moist air, carried on a prevailing wind, is lifted by an elevated landform, such as a mountain range, to a level where condensation takes place. Among the most common orographic clouds is orographic stratus. This low-level formation occurs most frequently in areas, such as coastal regions, where the air flow is heavily moisture laden. Generally, the landform must be at least 500 feet high to generate cloud, and higher in areas of clean, dry air, such as deserts.
Unlike normal stratus, which is carried about by the external wind field, orographic stratus tends to remain stationary. The wind flows through the area of condensation, constantly regenerating cloud as the air rises and dissipating it as the air descends on the other side of the landform.
The extent of this type of cloud depends on the humidity of the surrounding air mass. If the air contains a high level of moisture, cloud can begin to form well down the windward slope of the landform, wrap around the peak, and extend some distance down the other side. A good example of this type of formation is the "tablecloth" that often drapes the top of Table Mountain near Cape Town in South Africa.
The extent of the cloud is also determined by the steepness and elevation of the landfornn, the strength of the wind, and the direction of the wind relative to the landform. Strong wind blowing at right angles to a steep mountain will create greater uplift and generate more cloud.
Since low-level moisture and high land are essential ingredients in the formation of orographic stratus, areas that have high humidity and steep terrain -- tropical islands such as Hawaii, for example -- are particularly conducive to the formation of these clouds.
Sometimes, there is insufficient moisture in the lower layers of the atmosphere to allow condensation at ground level, but with increasing altitude and lower temperatures, condensation may be possible. In this case, lifting of the air mass by a landform can produce middle-level orographic formations, known as lenticular clouds.
One of the most spectacular forms of orographic stratus occurs in a variation known as banner cloud (or cap cloud), which can form on the peaks and immediately to the lee side of large mountain ranges. In this case, the cloud producing mechanism is a little different.
When wind blows over a tall, steep mountain, a certain amount of air stalls and piles up on the windward side, resulting in an area of increased air pressure. This, in turn, results in a drop in air pressure immediately to the lee side of the mountain. Since low pressure enhances condensation, a cloud will readily form in this position if sufficient moisture is present.
Two celebrated examples of banner cloud stratus are the mighty plumes that sometimes trail the peak of Mount Everest, in the Himalayas, and the Matterhorn, in the Swiss Alps. However, this process may also be seen in a weaker form near lesser mountain peaks.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
