Dust devil
A whirlwind is a small-scale vortices with vertical axis usually lower height extension, which is limited to the atmospheric boundary layer. In contrast to Großtromben ( tornadoes ) there is no direct connection with convective clouds.
Depending on their appearance and the fluidized material different names are used: Heuteufel, Al Hul ( Bedouin ), Fog Devils, Dust Devil, sand devil Staubtrombe, Sandtrombe, dust and sand pants pants. The last two terms are misleading, however, because they can be confused with wind and water pants, but where is Großtromben. A special form of the whirlwind is the gustnado ( gustnado ).
Condition for the formation of a ground- whirlwind is overheating of the atmosphere. In this dry unstable stratification thermal bubbles come off the ground, which can focus an existing weak rotation of the air by stretching of the vortex in rapidly rising. Due to conservation of angular momentum it takes the wind speed by the pirouette effect rapidly and can in extreme cases reach hurricane strength.
An example of a violent dust devil is available from the United States. On September 14, 2000 swept across the Coconino County Fairgrounds in Arizona a Sandtrombe with estimated wind speeds of 120 km / h Several people were slightly injured, the buildings located in the trajectory of the vortex were slight damage as covered Plan, torn off roof tiles and bent metal rod on.
Meanwhile, wind gusts were measured up to just over 150 km / h in the course of Sandtromben already, maximum 200 km / h seem possible at the current state of science. In sum, the most whirlwind, however, are rather weak and depend only rarely damage. In an average dust devil only gusting to 50 km / h can be achieved.
The expansion of a Sandtrombe is very different - diameter of 0.5 m to about 200 m wide and several hundred meters in height, all sizes are possible. Occasionally, dust devils in associations or groups, such as on a squall line.
The life of a whirlwind is between a few seconds to about a half hour. You can remain almost stationary or moving at walking pace or, in extreme cases, up to 100 km / h forward.
The direction of rotation of dust devils is not determined due to the small spatial extent of the wind by the Coriolis force, wind direction and orography play a role here. Whirlwind occur there most where dry unstable conditions are achieved by strong sunlight, so especially in desert areas. In our latitudes, they are most likely in the warm season over open land to be found ( unvegetated fields, mowed lawns, sports fields, wasteland, slope areas above the tree line ). The special form of Gustnados is dynamically triggered at gust fronts before showers or thunderstorms. A rare variant at low air temperatures can be observed on (relatively) warm water surfaces as fog devil.
Dust devils occur not only on Earth, but were also observed on Mars.