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 desert dust for the dust to be visible without using a microscope.
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.
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.
The rain dust is very alkaline. 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. 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%).
- 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 grams per m2 (in a study made in Montseny, Catalonia) 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.
- Radioactivity from the Chernobyl disaster was carried by rain dust to Greece in 2000.
Rain dust is the most common cause of blood rain.
Red rain is however not always rain dust, see for example the Red rain in Kerala.
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