Precipitation

Phenomenology: Micro-bursts are transient (1 to 5 min), strong, locally confined (around 4 km in diameter) downwards winds, which seem to fall from clouds. The winds of a micro-burst can be recognized by falling rain curtains which horizontally spread at the ground and possibly rise up again in form of eddies. The process is schematically described in Scheme.jpg. Micro-burst winds are generated when falling precipitating air masses are heavily cooled down by evaporating hydrometeors, then touch the ground and are thereby forced to move horizontally. The winds of the micro-burst can reach top speeds up to 200 km/h, so that they are enormously dangerous for air traffic. Aircrafts which are approaching an airport for landing below thunderclouds can even be pressed down to the ground by micro-bursts.

Microburst1.jpg to Microburst3.jpg might show a micro-burst, hence in Microburst1.jpg the rain curtain near the ground seems to spread horizontally in parallel to its bottom line. However, because of the large distance this cannot be determined for sure. The situation is more distinct in the series of five images at the bottom.

Below a blast rotor (without precipitation, but with strong turbulences) a clearly noticeable micro-burst was formed (Microburst4.jpg and Microburst6.jpg) which lasted for several minutes. Unfortunately no further images could be taken, because the system moved a bit and it was only allowed to take pictures in certain directions at the military airport.

Microburst1.jpg to Microburst3.jpg: S. Borrmann, flight Varig RG2375 from Fortaleza to Sao Paulo Brazil, 2 March 2004 at 4:50 p.m.

Microburst4.jpg, Microburst6.jpg,
Rotor1-6.jpg: S. Borrmann, Darwin, Northern Territory, Australia, SCOUT-O3 Field Campaign 25 November 2005, 4:33 p.m. to 4:38 p.m. local time

Camera Parameters

Olympus E-1 SLR with a 14-54 mm objective. Exposure and sensitivity choice (ASA 100) were performed by the camera. The colour range was set to sRGB with a resolution of 2560 x 1920 Pixel x 24 colours.