The leaves of this
Schefflera show the dust marks left by rain dust (near Paris, France)Air pollution often causes rain to leave stains of dust after it evaporates, in
Monterrey, Mexico.
Rain dust or snow dust, traditionally known as muddy rain, red rain, or coloured rain, is a variety of
rain (or any other form of
precipitation) which contains enough
mineral dust, from
soils (particularly from
deserts), for the dust to be visible without using a microscope.
History
The rain dust phenomenon was studied by Italian scientist
Giuseppe Maria Giovene (1753–1837), who managed to correctly explain the phenomenon as early as 1803. On 7 March 1803, rain dust fell over
Southern Italy's region
Apulia. At that time, people believed that the rain was caused by the explosions of Italy's volcanoes
Mount Vesuvius or
Etna, or that it was due to the transport of matter coming from the sea floor and raised by vapor. Giuseppe Maria Giovene related the phenomenon to the wind which occurred prior to the rain event, and he came to the conclusion that the sand had come from Africa and that it had been pushed by the wind coming from south-east.[1][2]
Geography
Rain dust is common in the Western and Southern Mediterranean, where the dust supply comes from the atmospheric depressions going through the northern part of North Africa. The main sources of desert dust reach the
Iberian Peninsula and the
Balearic Islands in the form of dust transported by wind or rain from the
Sahara,
Atlas Mountains in
Morocco and Central
Algeria.[3]
Mud rains are relatively frequent and had been increasing in early 1990s in the
Mediterranean Basin.[4]
The rain dust is very
alkaline.[3] Some of the large particles contain mixtures of chemicals such as sulfate and sea salt (chiefly with sodium, chlorine and magnesium). Major minerals in order of decreasing abundance are:
illite,
quartz,
smectite,
palygorskite,
kaolinite,
calcite,
dolomite and
feldspars.[3] In
Majorca a study finds that the size, by volume, 89% of the particles from rain dust fraction corresponded to
silt (between 0.002 mm and 0.063 mm) and that there was virtually no
clay sized particles (less than 0.29%).[5]
Importance
The particulates that rain dust carries are important for the formation of long-term
soil counteracting, in large part, the effects of
soil erosion. The amount of solids in rain dust have been estimated at 5.3 g m-2yr-1 (in a study made in
Montseny,
Catalonia)[6] in this location the dust provides 34% of the calcium needed by the
holm oak. The amount of the deposition of dust particles is highly variable depending on the year.
Saharan dust significantly increases the
pH of rain water. This may counteract the effects of
acid rain.
^Papastefanou, C; Manolopoulou, M; Stoulos, S; Ioannidou, A; Gerasopoulos, E (2001). "Coloured rain dust from Sahara Desert is still radioactive". Journal of Environmental Radioactivity. 55 (1): 109–112.
doi:
10.1016/S0265-931X(00)00182-X.
PMID11381550.