Description

The levant is an easterly wind that blows in the western Mediterranean Sea and southern France, an example of mountain-gap wind. In the western Mediterranean, particularly when the wind blows through the Strait of Gibraltar, it is called the Levanter. It is also known as the Solano. When blowing moderately or strongly, the levant causes heavy swells on the Mediterranean. Usually gentle and damp, the levant frequently brings clouds and rain. When it brings good weather, it is known as the "levant blanc" or "levante calma" in Gibraltar.

The Strait of Gibraltar

The Strait of Gibraltar, located at the western entrance to the Mediterranean Sea, is frequently associated with strong gap winds. The most pronounced gap wind through the Strait, the Levanter, can produce winds of 20 to 40 kts in and to the west of the gap when there is higher pressure to the east, over the Mediterranean, with lower pressure to the west of Gibraltar. The sinking motions accompanying such anticyclonic conditions cause stability in the low-level air flow, strongly suppressing vertical air motion and may result in the formation of an inversion within a few thousand feet of the surface. Such an inversion provides a cap that contains the low-level air and results in greater topographic blocking and the acceleration of the airflow through the gap forming the Strait. Under such circumstances, the winds can go from a moderate or fresh easterly over the Alboran Sea (the western part of the Mediterranean) to gale force strength on the western side of the Strait and to its west. Because the flow is accelerating and there is often a significant pressure gradient through the Strait, the strongest winds are not observed mid-Strait, as might be expected if the funnel mechanism was dominant; rather, the strongest winds are in the western Strait and immediately downwind to the west. Levanters are most frequent during the warm season from April until October and often reach a peak in spring, when the Mediterranean is comparatively cool, increasing the stability of the low-level air flow.

Levanter cloud

Sometimes the levanter forms a characteristic cloud named "banner-cloud" over the Rock of Gibraltar. However, this is not always the case and a particular set of conditions is required for its formation. Near the surface, the levanter is moist, but is unsaturated. As the moist air, which must be capped to be stable and so unable to rise by convection, is forced to rise over the Rock, the moisture condenses to form a cloud that streams away west from its top. If wind speeds are too low and stability high in the near-surface layer, the cloud does not form and condensation is also sensitive to small changes in moisture content, such that when the wind across the Rock veers into the southeast, the flow becomes too dry for the cloud to form, bringing drier air from North Africa. When the wind speed is too low, the air is blocked and unable to rise to form the cloud. At high wind speeds, the turbulent mixing to the lee of the Rock distributes the moisture through a comparatively deep layer and the cloud is, at best, very broken. Often it dissolves immediately west of the Rock in these turbulent windy conditions. In suitable conditions, the characteristic "pennant" cloud forms downwind. It usually extends about 3 nm west from the top of the Rock in a turbulent plume. This cloud hangs over the centre of the city of Gibraltar, while there is usually sunny weather in to the north and south from the southern outskirts of the city. 

Levanter cloud over he Rock of Gibraltar with prevailing easterly wind [Author: Nol Aders, Source: Wikicommons, June 2008.

Strong winds across the Rock 

When winds are very strong across the crest of the Rock, usually in excess of 30 kts, the cloud becomes detached from the crest of the Rock forming west of a line parallel to the ridge up to about 300 ft from it. At the same time, curved arcs of cloud may be seen within or below the pennant cloud, indicating the formation of a roll cloud. Usually, this produces light and rather variable winds near sea level, at times forming a cyclonic circulation area over Gibraltar Bay and the town. However, at times, the strong winds break away from the crest of the Rock, producing gusts to about the speed of the wind over the crest. These winds are usually northeasterly or easterly and may be several Beaufort forces stronger than the mean wind before and after.

Flight safety considerations

Strong winds across the Rock can cause turbulent conditions to the lee of the rock. When the wind is from the south, strong crosswinds cause unstable conditions on approach to the runway, which is situated to the north of the Rock.