Cumulonimbus Clouds (Cb)
Microphysics
Cb Calvus
Cb Inflow
Cb Hector I
Cb Hector II
Cb Calvus Capillatus
Development
Cb above Taunus
Cb from Aircraft
Cb in the Tropes
Views of Tropical Cb
Cb Mammatus I
Cb Mammatus II
View from Aircraft
Cb Incus
Cb - Ci I
Cb - Ci II
Squall lines from Cb
Overview

 

 


Squall line of Cumulonimbus Clouds


Definition and Properties: A "Squall line" is a more or less straight alignment of thunderstorm cells, high Cumulus or Cumulonimbus clouds. It is a linear arrangement of convection, shown orthogonally to the image plane of "SquallLineScheme.jpg". In connection with squall lines often strong winds of short duration occur. These typically fast moving "lines" can reach lengths between 20 and 1500 km. They can be part of a "Mesoscale Convective System (MCS)". The "Squall lines" are often located directly in front of cold fronts in a distance of 150 to 500 km with parallel alignment.

In the tropes the formation does not go along with cold fronts. Another way is the generation of squall lines from several, single thunderstorm cells or spiral bands of hurricanes that merge together. Squall lines are predominantly found in the tropes or in tropical air masses. This is because they only exist when humid air continuously flows into the cloud band and at the same time behind them cold air descends. This flow system is described in SquallLineScheme.jpg. Such conditions occur for example at the African Monsoon (humid air from the ground) and the African Easterly Jet (AEJ). The squall lines are responsible for short, strong precipitation (including hailstones). A clear air mass can be often observed before their arrival. The associated ground winds are straight, not rotating, from a constant direction.

The images were taken at Morro Branco, approximately 100 km southeast of Fortaleza at the Brazilian Atlantic Coast shortly after sunrise. At the top the squall line is presented as a linear arrangement of Cumulonimbus systems. Three Cb towers can be recognized in behind each other. A fourth one was above the observer, beyond the horizon there could be even more. The cloud bases were all at the same level at the "lifting condensation level". The image in the second row shows the falling rain of the front Cb tower in detail. In the last three images views from the neighbourhood of the photographer in different directions are shown. The squall line was surrounded by various layered, cumulus and rotor clouds in a wide range. The strong choppy wind which is characteristic for squall lines could be also here clearly observed.

Florida.jpg:
This image was taken from the Space Shuttle. The caption released by NASA was: This spectacular, low-oblique photograph shows a convective line of thunderstorms associated with a passing cold front over Florida. A shadow from the height of the thunderstorms, caused by early morning sunlight, can be seen traversing the scene southwest to northeast. The clouds in the storm system rise to about 16,500 meters (55,000 feet).

 

The V-shaped cloud structure is normally associated with cold fronts that cross the Gulf of Mexico and Florida in late winter and early spring. Severe thunderstorms and tornadoes usually occur with this type of storm system. At the time this photograph was taken, weather stations across Florida reported severe thunderstorms, strong winds, hail, torrential rains, and numerous tornadoes.


SquallLineScheme.jpg: Dissertation of Andreas Reiner, University at Cologne,Germany, 2003

SquallLine1-13.jpg: S. Borrmann, Morro Branco near Fortaleza, Ceara, Brazil, 24 February 2004, 6:26 to 6:42 a.m.

Florida.jpg: NASA Space Shuttle Mission STS41C-40-2130, NASA Copyright Free Policy, April 1984

Camera Parameters

Olympus E-1 SLR with 14-54 mm objective. Exposure and Sensitivity choice (100 ASA) was performed automatically by the camera. The images were taken in the sRGB colour range with a resolution of 2560 x 1920 Pixel x 24 colours. Since the whole scenery was very dark, the shot noise of the CCD elements became slightly visible. It could be reduced by the noise reduction function of the camera.

 
Lens aperture
Time of exposure
Focal length
SquallLine1.jpg

5.6

1/200

27

SquallLine3.jpg

5.6

1/320

54

SquallLine4.jpg

6.3

1/200

14

SquallLine5.jpg

6.3

1/200

25

SquallLine7.jpg

6.3

1/200

14

SquallLine9.jpg

5.6

1/250

38

SquallLine10.jpg

4.5

1/160

37

SquallLine13.jpg

5.0

1/200

31