Small currents
A small current is a small-scale air vortex with a vertical axis and mostly low elevation, which is limited to the atmospheric boundary layer . In contrast to large currents (tornadoes) there is no direct connection with convective clouds.
Depending on their appearance and the material blown up, different names are used: Today devil, Al Hul (with Bedouins), foggy devil, dust devil, sand devil, fire devil , dust bomb, sand bomb, dust pants and sand pants . The last two terms are misleading, however, as they can be confused with tights and waterspouts, which are large turbines. Snow devils also rarely appear. Gustnados (gust front vortices), however, should not be confused with dust devils.
The condition for the formation of small explosions is an overheating of the atmosphere close to the ground. In this dry unstable stratification, thermal bubbles detach from the ground, which can concentrate an existing weak rotation of the air by stretching the vortex during rapid ascent. Due to the conservation of angular momentum , the wind speed increases rapidly due to the pirouette effect and in extreme cases can reach hurricane strength.
An example of a violent dust devil is from the USA . On September 14, 2000, a sand storm swept over the Coconino County Fairgrounds in Arizona with wind peaks estimated at 120 km / h. Several people were slightly injured, and the buildings in the vortex's trajectory suffered minor damage such as covered tarpaulins, torn roof tiles and bent metal rods.
In the course of sand drifts, wind peaks of just over 150 km / h were measured, a maximum of 200 km / h seems possible according to the current state of science. Overall, however, most small currents are quite weak and rarely cause damage. In an average dust devil, only wind peaks of 50 km / h are reached.
The expansion of a sand stream varies greatly - from 0.5 m in diameter to around 200 meters in width and several hundred meters in height, all sizes are possible. Occasionally, dust devils also appear in associations or groups, for example on a gust line .
The service life of a small current is between a few seconds and around half an hour. You can stay almost stationary or move forward at walking pace, or in extreme cases up to 100 km / h.
The direction of rotation of dust devils is not determined by the Coriolis force due to the small spatial extent of the wind ; wind direction and orography play a role here. Small currents occur most frequently where dry-unstable conditions are achieved by strong solar radiation, especially in desert areas. In the middle latitudes, they are most likely to be found in the warm season over open land areas (ungrown fields, mown meadows, sports fields, wasteland, slopes above the tree line ). The special form of the gustnado is triggered dynamically on gust fronts before showers or thunderstorms . A rare variant at low air temperatures can be observed as fog devils over (relatively) warm water surfaces .
Dust devils are not only found on Earth , but have also been observed on Mars .
Individual evidence
- ↑ "Schneeteufel" in Lungau - salzburg.ORF.at. In: orf.at. salzburg.orf.at, accessed on April 3, 2015 . With video.
- ↑ Press Release Images: Spirit. NASA, July 12, 2007, accessed May 25, 2011 .
- ^ The Serpent Dust Devil of Mars , accessed on March 9, 2012.
- ↑ NASA: 12-Mile-High Martian Dust Devil Caught in Act. ( Memento of the original from May 3, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
Web links
- Everything about dust devils from Thomas Sävert