Picture of the Day: The Rock of Gibraltar Levanter Cloud
Seen here is a levant cloud forming against the eastern cliffs of the Rock of Gibraltar. 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 Viento de Levante or the Levanter. [source]
The Rock of Gibraltar is a monolithic limestone promontory located in the British overseas territory of Gibraltar, off the southwestern tip of Europe on the Iberian Peninsula. It is 426 m (1,398 ft) high and borders Spain. Most of the Rock’s upper area is covered by a nature reserve, which is home to around 250 Barbary macaques. These macaques, as well as a labyrinthine network of tunnels, attract a large number of tourists each year. [source]
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 which 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 5 km west from the top of the Rock in a turbulent plume. (Similar clouds may sometimes be seen elsewhere – notably the pennant cloud that forms on the Matterhorn in Switzerland.) 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.
You can read more about the unique cloud formation here.