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After-effects of splashing rain: insights from high-speed imaging


Rainfall can have several types of after-effects. Research on the phenomenon of bioprecipitation is based on the idea that some of these after-affects feed back into the processes involved in rainfall formation [1]. The most crucial for bioprecipitation is the generation of aerosols containing ice nuclei. Enhanced atmospheric concentrations of ice nuclei immediately after rain, and also for prolonged post-rain periods have been observed under various conditions [2, 3, 4]. Biological particles are among the aerosols generated by rain. For example, the concentration of airborne biological particles and ice nuclei in a forest eco-system increased about 10-fold during rain and up to one day thereafter in periods of extended leaf wetness [4], and the increases in ice nuclei were associated with increases in atmospheric concentrations of bacteria and fungi. Forceful impact of rain on leaves leads to rapid increases in leaf-surface population densities of the ice nucleation active bacterium Pseudomonas syringae [5] by a mechanism that has not yet been elucidated. These bacteria could contribute to atmospheric concentrations of ice nuclei when emitted into the atmosphere via the processes of aerosolization. Rain could also cause enhanced aerosol concentrations by what is generally known as splashing. Via high speed imaging, a team from the Dept. of Mechanical Engineering at MIT have provided details of aerosol generation by splashing in the case when raindrops fall on a porous surface – and on soils, in particular [6]. This team of researchers has suggested that this could be a means of release of microorganisms associated with soil into the atmosphere. But it could also provide a mechanism for soil particles themselves to become aerosols, and perhaps most importantly particles from rich organic soils, where plants are growing under conditions with sufficient rainfall, thereby increasing the atmospheric concentrations of the efficient ice nuclei associated with organic soils [7, 8].

A video of their observations can be seen at:

In reading background information about this work, I came across the history of the discovery of the nature of petrichor, the earthy smell that often follows rain [see: the original article; popular press information]. The Australian research team that discovered the chemical nature of this compound was interested in its effect on plant growth. Very unexpectedly, they found that petrichor was inhibitory to seed germination [9] – a seeming paradox if rainfall is to have a beneficial effect on sprouting of plants after dry conditions. Oddly, this work has never been followed-up (based on the lack of Web of Science citations relative to plant biology), leaving open some questions about other after-effects of rainfall.


  1. Morris C.E., Conen F., Huffman J.A., Phillips V., Pöschl U., Sands D.C. 2014. Bioprecipitation: A feedback cycle linking Earth history, ecosystem dynamics and land use through biological ice nucleators in the atmosphere. Global Change Biology 20:341-351
  2. Bigg E.K. 1958. A long period fluctuation in freezing nucleus concentration. J. Meteorol. 15:561-562
  3. Bigg E.K., Miles G.T. 1964. The results of large-scale measurements of natural ice nuclei. J. Atmos. Sci. 21:396–403
  4. Huffman J. A., et al. 2013. High concentrations of biological aerosol particles and ice nuclei during and after rain. Atmos. Chem. Phys. 13:6151–6164
  5. Hirano S.S., Baker L.S., and Upper C.D. 1996. Raindrop momentum triggers growth of leaf-associated populations of Pseudomonas syringae on field-grown snap bean plants. Applied and Environmental Microbiology 62:2560–2566
  6. Joung Y.S., Buie C.R. 2015. Aerosol generation by raindrop impact on soil. Nature Communications 6:6083 doi: 10.1038/ncomms7083
  7. Conen F., Morris C.E., Leifeld J., Yakutin M.V., Alewell C. 2011. Biological residues define the ice nucleation properties of soil dust. Atmos. Chem. Phys. 11: 9643-9648
  8. O’Sullivan D. et al. 2014. Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components. Atmos. Chem. Phys. 14:1853-1867
  9. Bear I.J., Thomas R.G. 1965. Petrichor and plant growth. Nature 207:1415-1416.
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