Cumulus Clouds (Cu)

Flight AA 8861 went from Miami over the Bahamas and the Dominican Republic to San Juan in Puerto Rico at a cruising altitude of 9600 m. Thereby a cross-section of clouds could be observed at the upper limit of the marine boundary layer in the northern Caribbean during one morning. The twenty-eight pictures which are presented here were chosen from a series of 135, taken during the 2 ½ hours of the flight. All clouds are Cumulus clouds of the kind of humilis and mediocris (e.g. MABL23. jpg). The last images present layers of Stratocumulus clouds.

All images seem to be "milky", because there was a haze layer from the ground to the cruising altitude during the whole flight with varying optical density.

This can be clearly seen in MABLClouds8.jpg or MABL14.jpg. According to the pilot's information the cloud base was in Puerto Rico at around 1500 m (MABL27.jpg and MABL28.jpg). Most probably it was nearly the same for the clouds in the other images.

In the series MABL4.jpg to MABL6.jpg the clouds are arranged in a regularly pattern of a cloud street. This phenomenon occurring in the planetary boundary layer is described in more detail in chapter Cloud Streets.

Clouds reach larger altitudes above islands compared to the high sea (e.g. MABL13.jpg and MABL22.jpg). Also in this series clouds are often observed above land, even if the islands are small, whereas larger oceanic regions

remained cloudless. This is due to the fact that the land areas are warmer than the ocean and therefore the convection is stronger. In the humid marine atmospheric boundary layer this convection always causes cloud formation.

MABL0-28.jpg: S. Borrmann, flight from Miami to San Juan, Puerto Rico, from 2 April 2005, 8:15 a.m. (local time Miami) till noon (local time Puerto Rico = local time in Miami plus 1 hour).