Cumulus Clouds (Cu)

Streets1.jpg to Streets5.jpg:
Extended fields of Cumulus clouds which are very regularly grouped in long-spread streets can be noticed sometimes from aircrafts or satellites. This is especially the case for regions of the trade winds as shown here. Four prerequisites have to be met for the formation of such cloud streets.
1. The earth's surface must be homogeneously heated up over large areas.
2. Relatively strong and steady horizontally winds have to occur from a single direction.
3. An inversion located above the scene.
4. The air humidity has to be sufficiently high and the altitude of the condensation level is within the boundary layer.

Since the surface temperature of the ocean is constant over a wide range and the solar radiation is homogeneous, the first assumption is often true, because temperature gradients are rarely generated at the sea surface. In the region of the trade winds steady and unidirectional winds exist and frequently an inversion at the top of the marine boundary layer is formed. Due to the warming of the surface the air is lifted, whereby it is "orthogonally blown" by the prevailing horizontal wind. Then horizontally arranged roll vortices are formed which are parallel to the prevailing wind direction. These long "cylinders" rotate around their axis parallel to the surface of the ocean.

This generates weak helical circulations of the air parcels that move around the cylinder in a spiral path from the top to the bottom. The tangential velocities of the flow along the curved surfaces are in the range of 1 m/s and are hard to measure directly. Two vicinal rolls rotate like two gearwheels grabbing into each other in opposite directions, i.e. one clockwise and the other one counter-clockwise. The diameter of the rolls is similar to the altitude of the marine boundary layer. By this means convectively ascending air bubbles ("plumes" or "thermals") which would be lifted vertically without horizontal winds are arranged in parallel rows within the boundary layer.

Strong anabatic winds occur at the interface between two surfaces of adjacent rotors where the velocity vectors point upwards. In the case of sufficiently high humidity - what is often the case above the warm ocean - and a lifting condensation level within the boundary layer, cloud bands are formed below the upper limit of the marine boundary layer between two vicinal rotors. The vertical extension of the clouds is thereby determined by the altitude of the condensation level and the temperature inversion at the top of the boundary layer. So the clouds of the street have mostly similar sizes.

These kinds of cloud streets are also formed as a result of advection of cold air over warm water surfaces. This is demonstrated in chapter Isobaric Mix Clouds. Above the oceans these cloud streets are often slowly converted into cellular arrangements.

Spin1-5.jpg: The spiral arrangement of the Cumulus humilis and Cumulus mediocris clouds presented here is remarkably. It is not clear how such an arrangement is formed. Perhaps it is a part of a von Karman vortex street which formed at the lee of a mountainous island.

Spin6.jpg: This satellite image proves that such vortex streets really exist.

Original Caption Released with Image:
Marine stratocumulus clouds frequently form parallel rows, or "cloud streets", along the direction of wind flow. When the flow is interrupted by an obstacle such as an island, a series of organized eddies can appear within the cloud layer downwind of the obstacle. These turbulence patterns are known as von Karman vortex streets. In these images from NASA's Multi-angle Imaging SpectroRadiometer, an impressive vortex pattern continues for over three hundred kilometers southward of Jan Mayen island. Jan Mayen is an isolated territory of Norway, located about 650 kilometers northeast of Iceland in the north Atlantic Ocean. Jan Mayen's Beerenberg volcano rises about 2.2 kilometers above the ocean surface, providing a significant impediment to wind flow.

These MISR images were captured on June 6, 2001, during Terra orbit 7808. The entire vortex street can be seen in the top panel, which is a natural-color view from the instrument's nadir (downward-looking) camera. The area covered measures 365 kilometers x 158 kilometers, and a cloud-clearing effect is apparent at the vortex centers until finally closing on the sixteenth "hole". The bottom panel is a stereo anaglyph of a portion of the vortex street, compiled using data from MISR's 26-degree forward and 70-degree backward viewing cameras. This view covers an area of about 183 kilometers x 96 kilometers. Despite the vertical exaggeration afforded by using widely separated angles, the relatively modest height variation in the cloud layer implies a vertically stable atmosphere. To facilitate stereo viewing, the images have been oriented with north at the left. Red/blue glasses should be used with the red filter placed over your left eye.Information on ordering glasses can be found here.

Fluid dynamicist Theodore von Karman was the first to derive the conditions under which these turbulence patterns occur. Von Karman was a professor of aeronautics at the California Institute of Technology and one of the principal founders of NASA's Jet Propulsion Laboratory.
MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The Terra satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of the California Institute of Technology.

Anticyclone.jpg: This image which was taken from the Space Shuttle shows that clouds can be arranged in large-scale, spiral patterns.

Original Caption Released with Image:
This pinwheel of anticyclonic clouds was photographed by the STS 41-B crew over the southern hemisphere of the Pacific Ocean. The ground winds at the center of this cyclonic system reach 80 kilometers per second (50 miles an hour). Circular storms in the northern hemisphere produce spiraling clouds with a clockwise pattern, while southern latitude storms have a counterclockwise cloud motion.

Streets1.jpg to Streets4.jpg: S. Borrmann, flight from Frankfurt to Lanzarote, 27 September 2003, around 6:30 a.m.

Streets5.jpg: S. Borrmann, flight from Frankfurt to Nice, 7 April 2003 7:48 a.m.

Spin1-5.jpg: S. Borrmann, flight from Tel Aviv, Israel, to Frankfurt, 3:30 p.m. MESZ, 2 June 2005

Spin6.jpg: Image from the Terra Satellite, Photojournal, 6 June 2001, Image credit and Copyright: NASA/GSFC/LaRC/JPL, MISR Team

Anticyclone.jpg: LPI/NASA, Space Shuttle Mission STS-41B, Cross Reference 11-37-1875, Copyright: LPI/NASA.