Castellanus1.jpg to Castellanus3.jpg: Cloud streets of Cumulus castellanus (former castellatus) off the coast of Venezuela which can be vaguely noticed in the background are presented here. These clouds are formed in stable layered and cloudless air - a situation that generally inhibits cloud formation. The buoyancy arises just because of condensation and not because of ascending warm air bubbles ("thermals"). Castellanus clouds are preferentially found in the lowest level above the ocean. Here the temperature gradient between sea surface and the air directly above is not significant so that there is no strong convection.

Further, the temperature of the sea surface is constant over large regions and only slightly varying. Therefore warm air bubbles are hardly generated, in contrast to situations on shore. There different surfaces (e.g. asphalt, forest) have extremely different temperatures after receiving the same amount of insulation and so the formation of thermals is supported. The surrounding air between individual towers is dry enough for fast evaporation of small cloud fragments. Since here buoyancy is not supported by the updraft of warm air bubbles, the Cumulus parts of the clouds are smaller than normal Cumulus, where already a significant upwards velocity is observed at the condensation level.

If the surrounding air of the clouds is very dry, Castellanus clouds or parts of them will quickly evaporate and Cumulus floccus clouds will be formed. In contrast to the larger Cumulus humilis or mediocris this type of clouds can be recognized at Castellanus1.jpg and Castellanus2.jpg near the engine suspension.

Castellanus4.-9.jpg:Here Stratocumulus castellanus clouds were pictured from the sea surface. This series was taken between 8:00 and 8:25 a.m. local time in Arrieta, in Northern Lanzarote. One can see the rising position of the sun. The brownish colouring of the lowest air layer is due to Saharan dust which was transported to the island during these days. More details can be found at the chapter "Sahara Dust".

Maledives.jpg:
"Cloud streets, Tiladumati Atol, Maledive Islands; Small cumulus clouds frequently arrange in parallel rows or "cloud streets" under stable air conditions. These cloud streets over the reefs of the Maldives in the Indian Ocean denote the prevailing wind direction, with the cloud streets lying parallel with the wind. Turbulent air lifted by the windward portions of the islands promotes cloud formation downwind. (STS 41-C, April 1984. Picture # 13-36-1621) "

In the case of these clouds which also are Stratocumulus castellanus the cloud formation in the stable atmospheric layering is triggered by a small lift of the air. The average height of the atolls belonging to the Maldives Archipelago is 2 m above sea level. This small interference is sufficient to initiate the condensation which leads to buoyancy and cloud formation as a consequence of released latent heat.

Castellanus1-3.jpg: S. Borrmann, flight from Frankfurt to Bogota, 15 September 2001, Atlantic Coast at Venezuela

Castellanus4-9.jpg: S. Borrmann, Arrieta in Northern Lanzarote, 26 September 2002, between 08:00 a.m. and 08:25 a.m.

Maledives.jpg from Pat Jones, Shuttle views the Earth: Clouds from Space, 1989 (Copyright: NASA)